Thursday’s announcement by North Korea that it could launch a pre-emptive nuclear attack against the United States in the face of new U.N. sanctions is a predictable escalation of the isolated nation’s increasingly aggressive stance toward Washington over the past year. But experts note that Pyongyang’s recent advances in its nuclear weapons and missile programs mean that such bellicose rhetoric cannot be taken lightly.

The escalation of the North’s oratory began not long after the country’s 28-year-old leader, Kim Jong Un,  took over from his late father, Kim Jong Il, on Dec. 28, 2011. It has been accompanied by two space launches – one successful – and a third nuclear weapons test.

 It is not unusual for the North to make threats against the U.S., Japan or South Korea. And on occasion -- as in the case of the 2010 artillery barrage of Yeonpyeong Island and an earlier attack on a South Korean gunboat -- it has carried out these threats.  It has never taken any military action after threatening the United States, however.

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Some analysts have suggested that the latest round of threats is intended to show that the young Kim will continue his father’s legacy of hostility toward the U.S.

To what end?

North Korea has long wanted the U.S. to sit down with its negotiators to hammer out an agreement to end the Korean War, which ended in 1953 not in a peace treaty but in a truce.

The North would like to gain concessions from the U.S. in such a negotiation, but its escalating threats and rhetoric have the opposite effect:  The Obama administration, like preceding administrations, has steadfastly refused to negotiate with Pyongyang.

The problem is that North Korea, which has long taken a backseat in U.S. councils to the Middle East, does have military capabilities that could at the very least threaten U.S. interests in North Asia.

According to a recent analysis, North Korea has a weapon stockpile that could threaten both Japan and South Korea and, in longer term, the United States. Some of the weapons have already been deployed, say U.S. officials, who spoke to NBC News on condition of anonymity. Moreover, the North has begun research into more advanced and dangerous weapons, possibly even thermonuclear weapons, they say. 

At the high end of the stockpile range, U.S. officials and other researchers said North Korea may already have up to "a few dozen" nuclear weapons that could be fitted atop its vast fleet of ballistic missiles. Those missiles are limited to an intermediate range, capable of hitting targets in Japan, South Korea or elsewhere in the northern Pacific, including U.S. military bases as far south as Guam, the officials believe.

Related story: UN passes sanctions despite North Korea threat of 'pre-emptive nuclear attack'

The U.S. believes the space launch tests are part of a development plan for an intercontinental ballistic missile (ICBM) capable of reaching the continental United States with a payload of several hundred kilotons — 10 to 20 times the size of the bombs that destroyed the Hiroshima and Nagasaki.

U.S. officials publicly express confidence that the national missile defense system based in Alaska would be able to shoot down any incoming North Korean ICBM.

“I can tell you that the United States is fully capable of defending against any North Korean ballistic missile attack,” White House spokesman Jay Carney said Thursday in response to a question about the North Korean threat.

 He also said the U.N. sanctions will make it harder for Pyongyang to continue to make progress on its weapons and missiles. 

“North Korea … will now face new barriers to developing its banned nuclear and ballistic missile programs,” he said. “Resolution 2094 increases North Korea's isolation and demonstrates to North Korea's leaders the increasing costs they pay for defying the international community.” 

For the past several years, the U.S. also has been monitoring North Korean research into thermonuclear weapons — hydrogen bombs and bombs known as boosted-fission weapons, in which plutonium and uranium are combined for a higher energy yield. (The problem is that if the North conducted a test and claimed that it was thermonuclear, the U.S. would have difficulty determining if the North was telling the truth. The test site at Kilchu is far enough inland that the U.S. would not have access to the particulate matter needed to make an accurate determination, experts say. )

David Albright, president of the Institute for Science and International Security, or ISIS, a nonpartisan nuclear arms research group, said last year that any tests in the future may also be about ensuring the reliability of North Korea's current weapons design.

"Once you get beyond a dozen, it makes sense to test type and reliability of your weapons," he said. Albright said then that his group's estimate of North Korea's weapons stockpile is a bit less than those provided by the U.S. officials, but that ISIS, too, believes Pyongyang has "missile-deliverable weapons."

The design of the weapons is believed to be based on Chinese models (as were the first generation Pakistani nuclear weapons). The design is basic, and was developed in the 1960s with help from the Soviet Union, which used it to produce a whole line of nuclear warheads.

While some analysts suggested that the North planned its December rocket launch to gain attention ahead of the presidential election in South Korea , some in the U.S. non-proliferation community think otherwise. They expect that once the North feels comfortable with its ICBM technology, it will deploy the missiles.  They point to the Musudan intermediate range missile which was tested in middle of the last decade, then deployed — presumably with nuclear warheads — and aimed at Japan.

Once the North has confidence in the long-range missile based on the space rocket, U.S. officials believe they will deploy it as well, making North Korea the third nation to have nuclear weapons targeted at the United States, after Russia and China.

Many in the Obama administration see that as a more frightening prospect than Iran gaining nuclear weapons, believing that Tehran is a rational actor that will serve its own national interest and preserve the regime, compared to successive generations of North Korean leaders who have shown that they are unpredictable and erratic.

But would it force the U.S. to conduct face-to-face talks with the North? State Department spokeswoman Victoria Nuland said in December that the North has a better option.

Referring to Kim Jong Un, Nuland said: "He can plot a way forward that ends the isolation, that brings relief and a different way of life and progress to his people, or he can further isolate them with steps like this. He can spend his time and his money shooting off missiles, or he can feed his people, but he can't have both."

NBC News' Shawna Thomas contributed to this report; this piece is an updated version of a post originally published on Dec. 13, 2012.

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North Korea does not appear to be making preparations for a nuclear weapons test following Tuesday’s test of a space launch vehicle, which was believed to be cover for a long-range missile test, U.S. intelligence analysts told NBC News.

South Korean and Japanese officials had feared that a nuclear weapons test — its third after previous detonations in in October 2006 and May 2009 — would quickly follow the launch.

But word that the North isn’t thought to be preparing for a test is providing little solace for Seoul or Tokyo, mainly because recent intelligence suggests that the North has made significant advances in its nuclear weapons program.

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According to a recent analysis, North Korea has a weapon stockpile that could threaten both countries and, in longer term, the United States. Some of the weapons have already been deployed, say U.S. officials, speaking on condition of anonymity. Moreover, the North has begun research into more advanced and dangerous weapons, possibly even thermonuclear weapons, they say.  

 

At the high end of the stockpile range, U.S. officials and other researchers said North Korea may already have up to "a few dozen" nuclear weapons that could be fitted atop its vast fleet of ballistic missiles. Those missiles are limited to an intermediate range, capable of hitting targets in Japan, South Korea or elsewhere in the northern Pacific, including U.S. military bases as far south as Guam, the officials believe.

'Highly provocative'
The U.S. believes the space launch test is part of a development plan for an intercontinental ballistic missile (ICBM) capable of reaching the continental United States with a payload of several hundred kilotons — 10 to 20 times the size of the bombs that destroyed the Hiroshima and Nagasaki.

State Department spokesperson Victoria Nuland hinted about that Wednesday, calling the launch "highly provocative" and a "threat" to regional security. The U.S. is "concerned that all of this launching is about a weapons program and is not about peaceful uses of space," she added.

More North Korea coverage from NBC News

For the past several years, the U.S. also has been monitoring North Korean research into thermonuclear weapons — hydrogen bombs and bombs known as boosted fission weapons, in which plutonium and uranium are combined for a higher energy yield. (The problem is that if the North conducted a test and claimed that it was thermonuclear, the U.S. would have difficulty determining if the North was telling the truth. The test site at Kilchu is far enough inland that the U.S. would not have access to the particulate matter needed to make an accurate determination, experts say. )

David Albright, president of the Institute for Science and International Security, or ISIS, a nonpartisan nuclear arms research group, said earlier this year that any tests in the future may also be about ensuring the reliability of North Korea's current weapons design.

"Once you get beyond a dozen, it makes sense to test type and reliability of your weapons," he said. Albright said then that his group's estimate of North Korea's weapons stockpile is a bit less than those provided by the U.S. officials, but that ISIS, too, believes Pyongyang has "missile-deliverable weapons."

The design of the weapons is believed to be based on Chinese models (as were the first generation Pakistani nuclear weapons). The design is basic, and was developed in the 1960s with help from the Soviet Union, which used it to produce a whole line of nuclear warheads.

ANALYSIS: 'Spoiled child' North Korea snubs key ally China with rocket test

While some analysts suggest that the North is using its space rocket launch to gain attention ahead of next week’s presidential election in South Korea -- and possibly to force talks with the U.S. — some in the U.S. non-proliferation community think otherwise. They expect that once the North feels comfortable with its ICBM technology, it will deploy the missiles.  They point to the Musudan intermediate range missile which was tested in middle of the last decade, then deployed — presumably with nuclear warheads — and aimed at Japan. 

Once the North has confidence in the long-range missile based on the space rocket, U.S. officials believe they will deploy it as well, making North Korea the third nation to have nuclear weapons targeted at the United States, after Russia and China.

Many in the Obama administration see that as a more frightening prospect than Iran gaining nuclear weapons, believing that Tehran is a rational actor that will serve its own national interest and preserve the regime, compared to successive generations of North Korean leaders who have shown that they are unpredictable and erratic.

But would it force the U.S. turn to conduct face-to-face talks with the North? Nuland said Wednesday that the North has a better option.

Speaking of the North’s 27-year-old leader Kim Jong Un, Nuland said: "He can plot a way forward that ends the isolation, that brings relief and a different way of life and progress to his people, or he can further isolate them with steps like this. He can spend his time and his money shooting off missiles, or he can feed his people, but he can't have both."

Robert Windrem is a senior investigative producer for NBC News.

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North Korea could carry out an underground test of a nuclear weapon as early as Tuesday night as the North's reclusive leadership dramatically tries to up the stakes with the U.S. and the West, U.S. officials told NBC News.

U.S. officials say North Korea may already have an arsenal between 12 and a "few dozen" far more advanced weapons, many more than generally believed.

The officials couldn't be specific on a date for the test, but they told NBC News they were "100 percent" certain there would be a nuclear test within the next two weeks or "at any time."

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Tensions between North and South Korea increased this week when Pyongyang threatened to turn Seoul into "ashes." While the North regularly issues such threats, the South seemed to be taking this round of threats more seriously by increasing its security.

U.S. and South Korean intelligence agencies have been monitoring test preparations at P'unggye-yok, the North Korean test site near the Chinese border, for the past several weeks. As new evidence of tunneling emerged, officials began to see Army Day celebrations scheduled for Wednesday (Tuesday night in the U.S.) as a possible date for the test.

It will be the first time the country's 29-year-old leader, Kim Jong Un, will get a chance to address the Korean People's Army as commander.

At the high end of the range, U.S. officials and other researchers said, North Korea may already have up to "a few dozen" nuclear weapons that could be fitted atop its vast fleet of ballistic missiles. Currently, North Korean missiles are limited to an intermediate range, capable of hitting cities in Japan or South Korea but not the United States. What the new test could reveal is an improvement in the type of weapons North Korea has.

For the past several years, the U.S. has been monitoring North Korean research into thermonuclear weapons, hydrogen bombs and bombs known as boosted fission weapons, in which plutonium and uranium are combined.

David Albright, president of the Institute for Science and International Security, or ISIS, a nonpartisan nuclear arms research group, said Tuesday that the tests may also be about ensuring the reliability of North Korea's current weapons design.

"Once you get beyond a dozen, it makes sense to test type and reliability of your weapons," he said.

Watch the most-viewed videos on msnbc.com

Albright said that his group's estimate of North Korea's weapons stockpile is a bit less than those provided by the U.S. officials but that ISIS, too, believes Pyongyang has "missile-deliverable weapons."

North Korea successfully tested nuclear weapons in 2006 and 2009. In both cases, the first word came in statements from the North Korean Foreign Ministry hours before the tests were carried out. No such statement has been issued yet, but a U.S. official said it's possible that this time North Korea wouldn't follow the same protocol.

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Ten days ago, North Korea failed in its attempt to launch an observation satellite, a test the U.S. believed was a cover for test of intercontinental missile technology. In response, the U.S. canceled an agreement that would have provided 241,000 tons of nutritional aid, while the U.N. Security Council voted unanimously to "strongly condemn" the failed launch and said it would tighten sanctions against Pyongyang's government.

Albright added that North Korea might not want to test its weapons to their full yield in order to avoid another embarrassment, noting that the geology around the test site is fragile and that a large test could aggravate that issue.

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U.S. officials and others who track the nuclear capabilities and internal politics of North Korea say they don't see any indications that Pyongyang is planning an imminent nuclear test, but they caution that after the embarrassment of Friday's failed rocket launch it could move provocatively and quickly to do so.

"We consider it at any time a possibility," said one U.S. official who follows North Korea and who briefed NBC News on condition of anonymity. "Might kind of ruin the party or enhance it", he added, referring to Sunday's celebration of North Korean founder Kim il-Sung's 100th birthday.

The officials and experts who spoke with NBC News on Friday questioned whether the North would want to risk another, far greater embarrassment so quickly after the rocket failure. But if it does conduct a nuclear test, it will be following a long tradition of crisis escalation, they said. 

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The usual sequence in a North Korea crisis is threefold, said a second U.S. official, also speaking on condition of anonymity. First, the North does something untoward, and then the West protest, the official said. In response, North Korea does something to provoke even more reaction and get more attention, with the goal of ultimately driving the U.S. to the negotiating table. In this case that third piece could be the nuclear test, the official said. 

"What surprises me is how quickly this is moving. Things that used to happen in years (in North Korea) are now happening in months," said the second official. "When things start spinning fast, I don’t think that's stable, that's safe. So that's concerning.”

David Phillips of Columbia University's Institute for the Study of Human Rights and a North Korea expert, said the government of Kim il-Un does not have to "go nuclear." It has at least two options if it decides to make noise on the international stage: a renewed attack on South Korean islands or naval vessels, or a nuclear test, he said.

"One would be a serious provocation," Phillips said of the first option; the other would quickly become a "global issue."

U.S., South Korean and Japanese officials and experts note that there have been recent preparations at North Korea’s P'unggye-yok test site that could signal a nuclear test, but said those preparations have not intensified over the last few days.

David Albright of the Institute for Science in International Security said his analysts have been monitoring the site almost daily using commercial satellite imagery, but have detected only some movement of earth at a tunnel, which may or may not be related to a test.  Albright agrees that a test would quickly move the North's nuclear program to the forefront of global crises.

One thing that could fuel the crisis, he said, would be for the West, particularly Japan and South Korea, to ridicule the failure of Friday’s satellite launch. 

"It makes the North Korean military mad,” said Albright, who has visited Pyongyang and met with senior North Korean officials. “If they feel that they are now perceived as weaker, they may react to re-establish their deterrence capabilities."

That, he says, could lead to a further escalation of tensions in the region.

"It would further lock in the view that North Korea does not intend to give up nuclear weapons and it would greatly worry Japan, which always feels it is in the North Korean bull’s-eye,” he said. “Among the public in both Japan and North Korea, it would greatly stimulate the debate that they should get nuclear weapons."

Particularly worrisome for the U.S., senior security officials told NBC News, is the possibility that North Korea would test more sophisticated weapons designs – hydrogen bombs or so-called "boosted fission" weapons, both with yields that far exceed those of nuclear designs. Either a "boosted fission" weapon or a hydrogen bomb would be expect to have yields in the tens or hundreds of kilotons, or many times greater than the bombs the U.S. dropped on Hiroshima and Nagasaki, Japan, at the end of World War II.   

It is also possible the North Koreans could detonate a device fueled by highly enriched uranium (HEU) rather than plutonium, the officials said. . The North Koreans have used plutonium exclusively in the first two tests nuclear in 2006 and 2009.

Any of those tests -- HEU, "boosted fission" or thermonuclear-- would show that the North had more advanced weapons design and development capabilities, they said.

Indeed, U.S. officials said North Korea has done significant research into both "boosted fission" and thermonuclear weapons development in recent years. However, without testing, it couldn't be certain that such a weapon is reliable. One constraint, they said, would be whether the geology around the test site could withstand a test.

One U.S. official also said that U.S. intelligence might not be able to immediately confirm or contradict North Korean claims in the wake of such a test.

"If they do a very high yield test and get into multiple tens of kilotons, and they say it's thermonuclear, unless we have some kind of particulate sampling data, I'm not sure what we're going to say,” said one official. “And their statements could add to the confusion."

Phillips, the Columbia University expert, noted that a North Korean nuclear test could complicate U.S. proliferation priorities. What's more significant, he asked, an Iranian program that has been slogging along or a North Korean program with more than a dozen nuclear weapons, some of which that could have yields in the hundreds of kilotons.

“The Obama administration has been focused on Iran as the primary nuclear threat and proliferator. Many believe that Iran is a rational actor that will serve its own national interest and preserve the regime,” said Phillips. “The same can’t be said about North Korea. Successive generation of leaders in North Korea have shown that they are unpredictable and erratic. The recent satellite launch was designed to burnish the authority of its news leader. Instead its had the opposite effect internationally. ... There is now real risk of a nuclear test, which may now be accelerated by the launch failure and that’s the problem.”

And despite the failure of Friday’s rocket launch, U.S. officials say they expect North Korea to continue trying to develop a missile capability that could deliver a warhead big enough to destroy a U.S. city.

“The intelligence community has assessed for a number of years,” said the first U.S. official, “that this (launch) vehicle would be capable of reaching the continental United States -- beyond Alaska, beyond Hawaii --  with a payload of several hundred kilotons.”