U.S. officials have released new information accusing three high level Iranian Revolutionary Guard officials of overseeing an alleged plot to assassinate the Saudi Arabian ambassador. One of them, a deputy commander in the Iranian Qods Force, had previously been accused of plotting a highly sophisticated attack that killed five U.S. soldiers in Iraq, according to U.S. government officials and documents made public Tuesday afternoon.

The Qods Force official who coordinated the alleged plot was identified by the Treasury Department as Abdul-Reza Shahlai, the cousin of the suspect, Manssor Arbabsiar. Arababsiar was accused by U.S. law enforcement officials of seeking to carry out the plot to kill Saudi Ambassador Abdul al-Jubeir in Washington, D.C. and carry out other terrorist attacks in the U.S.

Three years ago, Shahlai -- the key Iranian official coordinating the attack -- was designated as a terrorist by the Treasury Department for fomenting violence in Iraq, including working with the anti-U.S. Mahdi Army to carry out a mass attack on U.S. soldiers in Iraq, according to Treasury documents.

In particular, he was accused of planning a Jan. 20, 2007, attack by Mahdi Army militia members aimed at U.S. soldiers in Karbala, south of Baghdad. In that attack, up to a dozen fighters with false IDs disguised themselves as an American security team to penetrate the provincial government building in Karbala and open fire. One U.S. soldier was killed in the initial attack and four others were abducted and found shot to death soon after.

Shahlai was not identified by name in the criminal complaint released by the Justice Department, referred to only as a "cousin" of the suspect, a high-ranking official in the Iranian Revolutionary Guard.

But on Tuesday the Treasury Department identified him and two other senior Iranian Qods Force officers as being involved in both the earlier attack and the plot to assassinate the Saudi ambassador on U.S. soil and imposed economic sanctions against them. The Treasury Department move significantly ratchets up the pressure against Tehran.

The senior Qods Force officers were identified as Maj. Gen. Qasem Solemami and Halem Abdollahi.

Solemami oversaw the Iranian officers involved in the plot, according to the Treasury announcement. Soleimani has twice been previously blacklisted by the department, most recently for allegedly overseeing Qods Forces in involved in human rights abuses against protesters in Syria.

Abdollahi allegedly coordinated aspects of the operation aimed at the Saudi ambassador, according to the announcement.

The Qods Force, an arm of the Iranian Revolutionary Guard Corps, is described by Treasury as the Iranian government's primary foreign action arm for support of terrorist organizations and extremist groups around the world. It is accused of providing training, logistical assistance and material and financial support to the Taliban, Lebanese Hezbollah and Hamas, among others. Its officers have also supported attacks against U.S. and allied troops and diplomatic missions in Iraq and Afghanistan, according to the Treasury announcement.

In a strongly worded letter to U.N. Secretary General Ban Ki-moon obtained by NBC News, Iran’s Ambassador to the U.N., Mohammad Khazaee, said Iran “strongly and categorically rejects these fabricated and baseless allegations.”

Accusing the U.S. of “warmongering,” Khazaee charged that U.S. authorities were carrying out an “evil plot in line with their anti-Iranian policy to divert attention from the current economic and social problems at home and the popular revolutions and protests against United States’ long supported dictatorial regimes abroad.”

Earlier Tuesday, Iran rejected U.S. claims that Tehran was involved in a plot to assassinate al-Jubeir.

Iranian Foreign Ministry spokesman Ramin Mehmanparast called the claims a "prefabricated scenario."

"These old-fashioned behaviors are based on the long-standing hostile American-Zionist policies and are ridiculous show in line with scenarios to provoke division," the semiofficial Fars news agency quoted Mehmanparast as saying.