A terror attack Monday on a Yemeni military parade rehearsal that killed scores occurred amid increasing cooperation between the Yemen and U.S. governments, with the latter stepping up assistance to the Yemeni military and regularly targeting purported terrorist cells with drone strikes.

The cooperation reflects a growing  belief in U.S. national security circles that al-Qaida in the Arabian Peninsula (AQAP), the Yemeni al-Qaida affiliate, is now a bigger and more dangerous threat than the central al-Qaida group in Pakistan. (AQAP on Tuesday claimed responsibility for the attack on the military parade and a shooting that targeted U.S. military trainers in the country. There were apparently no injuries in the second incident.)

The cooperation is not limited to counter-terrorism. The U.S. is openly helping the new Yemeni government in counterinsurgency efforts against an AQAP-affiliated group, Ansar al-Sharia, in the south of the country.  The assistance includes “a small contingent” of military trainers and intelligence officers assisting the Yemeni forces.

The presence of the American personnel in Yemen is raising concerns that Washington risks opening another front in the war against al-Qaida before it has fully extricated itself from long, bloody conflicts in Afghanistan and Iraq.

But a senior U.S. counterterrorism official, who spoke with NBC News on condition of anonymity, said the AQAP’s successes in recent months give Washington little choice but to increase support for the new Yemeni government of President Abd-Rabbu Mansour Hadi.

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“AQAP’s enhanced footprint in southern Yemen increases the chances that the group will establish a regional safe haven,” said the official. “This would be a dangerous development because AQAP’s anti-government fight and its terrorist plotting against the West are its two main goals. Unless its gains are reversed, AQAP will have more flexibility to conduct external attacks from a position of strength.”

The Yemeni government position is about survival. Like Tunisia, Libya and Egypt, Yemen is under new management, after former President Ali Abdullah al-Saleh’s replacement by Hadi, his former deputy, in February.

Hadi’s position is precarious. Ansar al-Sharia, an Islamist insurgent group, is trying to further destabilize the water-starved, tribal-riven state, with the ultimate goal of toppling his government.

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But Hadi’s  increasing reliance on U.S. help has likewise caused him some difficulties, triggering protests among middle- and upper middle-class Yemeni youth who are resentful over the U.S. role in the country, particularly the drone strikes and surveillance.

Mohammed Huwais / AFP - Getty Images

Yemeni military police collect evidence at the site of a suicide bomb attack in Sanaa on Monday, which killed nearly 100 members of a Yemeni army battalion

Michael Leiter, former director of the National Counter Terrorism Center and now an NBC News analyst, said the deaths of nearly 100 Yemeni soldiers in Monday’s bombing are likely to bring two countries’ counter terrorism efforts closer.

“Hadi's rise has probably brought greater legitimacy to cooperation with the U.S.,” said Leiter. “… The president (Hadi) and elements of the security and defense establishment cooperate with the U.S. but want to keep that relatively quiet in order to avoid enflaming the domestic population.  .. . And, frankly, with horrific attacks like today, U.S. assistance often becomes more rather than less welcome.”

A Yemeni official, also speaking on condition of anonymity because he is not authorized to speak with the press on security operations, contended that  “broad cooperation” with the U.S. is necessary and marks a “new level” of friendly relations between the two countries. He said the U.S. role in Yemen is limited in terms of numbers, but significant in helping the government turn back Ansar al-Sharia, which he characterized as “militants, drug dealers and foreign groups.”

'Intelligence, satellite images and technical advice'
“The U.S. is providing intelligence information, satellite images and technical advice” valuable in both  counterterrorism and counterinsurgency efforts, said the official. Both the U.S. intelligence community and the Joint Special Operations Command, JSOC, are involved. He emphasized there are “no boots on the ground” fighting with Yemeni forces.

Neither the U.S. nor Yemeni official would put numbers on the U.S. involvement. Nor would the Yemeni  deny the presence of CIA officers on the ground.

The most high profile product of this cooperation has been the drone attacks on both counterterrorism and counterinsurgency targets. High profile attacks have killed three top AQAP officials in the past eight months, but there also have been an increasing number of attacks on lesser figures and even suspected gatherings of terrorists. The attacks, said the Yemeni official, have taken place “all over the country.”

In September, apparently helped by material uncovered in Osama bin Laden’s compound in Pakistan, U.S. drones killed two American citizens in Kashef, about 85 miles east of the capital, Sanaa. The dead were Anwar al-Awlaki, an AQAP leader blamed for recruiting other Americans to the group’s violent cause, and Samir Khan, co-editor of “Inspire,” a magazine whose articles included “How to Make a Bomb in the Kitchen of Your Mom.”

Then, earlier this month, the director of AQAP’s external operations, Fahd al Quso, was similarly killed by a drone attack in a remote mountain valley -- his whereabouts reportedly exposed by a British-Saudi-U.S. undercover intelligence operation. The penetration of AQAP by an informant  also resulted in the interception of a new, more sophisticated  version of the underwear bomb previously used unsuccessfully  to try to down U.S. airliners. (Yemeni intelligence, said the Yemeni official, had “no role” in that operation and was unaware of it.)

By some estimates, the tempo of the drone strikes against AQAP and Ansar al-Sharia is now even greater than in Pakistan, with the number of attacks in May surpassing even the most intense month of attacks against al-Qaida central in Pakistan. According to the “Long War Journal” website, which uses local reporting to track Predator strikes, 30 AQAP fighters (and seven civilians) have been killed in five drone strikes in the past 10 days alone. (Both U.S. and Yemeni officials say that such local reports are often inaccurate or exaggerated.)

The larger concern in terms of U.S. involvement may be the counterinsurgency effort. The Los Angeles Times reported  last week that at least 20 U.S. Special Operations troops are using satellite imagery, drone video, eavesdropping systems and other technical means to help pinpoint targets for the Yemeni military offensive that’s currently under way in the south. 

The Yemeni official would only say that targeting is “very selective” and that “No Americans are fighting on the side of the Yemenis,” a point on which U.S. officials agree.

While the Yemeni official said the offensive has made great strides recently, there have been setbacks, including the killing of 32 Yemeni soldiers on May 7 when AQAP overran a Yemeni position. That was the deadliest single encounter for government forces in the war with AQAP until Monday’s attack.

The Yemeni official said that the public is supportive of both operations, despite a social media protest by the country’s youth that has drawn some attention.

He claimed that no civilians have been killed in the drone strikes and stated that that care has been taken to strike at times and places where only AQAP and its allies are present.

There’s no doubt that U.S. cooperation -- and the drone strikes—will continue. The U.S. wants to kill Ibrihim Hassan Al-Asiri, the AQAP’s expert bomb-maker, before he trains others in his craft. 

But the mere presence of U.S. military personnel in the country carries risk of a confrontation that could quickly escalate. This weekend, for example, a local Yemeni newspaper reported unidentified gunmen opened fire on a car that belonged to U.S. military trainers as they left the tourist al-Hodeida Land Resort in the western part of the country. None of the Americans were believed to have been injured.

Robert Windrem is a senior investigative producer for NBC News.

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