In the assault, up to a dozen gunmen posed as an American security team, with U.S. military combat fatigues, allowing them to pass checkpoints into the government compound, where they launched the attack. One U.S. soldier was killed in the initial assault, and the militants abducted four others who were later found shot to death.

On Monday, the U.S. military reported the deaths of five U.S. servicemembers killed in fighting a day earlier, including two soldiers who died in attacks in Baghdad and two soldiers and a Marine who died in fighting in western Anbar province. The deaths brought to 3,582 the number of members of the U.S. military who have died since the beginning of the Iraq war in March 2003.

The new accusations against Iran raise tensions between the two countries as Iraq is trying to organize a second round of direct talks between U.S. and Iranian officials in Baghdad. The U.S.-backed, Shiite-led Iraqi government, which has close ties to Iran, is pushing the two to ease their disputes to help reduce Iraq's turmoil, but a February meeting between the two sides made little headway.

Bergner said Iraqi extremists were taken to Iran in groups of 20 to 60 for training in three camps "not too far from Tehran." When they returned to Iraq, they formed units to carry out attacks, bombings and kidnappings.

"Our intelligence reveals that the senior leadership in Iran is aware of this activity," he said. He said it would be "hard to imagine" that Iran's supreme leader Ayatollah Ali Khamenei was unaware of the activity.

Hezbollah spokesmen in Lebanon said they were checking into the claims Dakdouk was a member of the group and would not comment. The group has in the past denied any activities in Iraq. In late 2005, British Prime Minister Tony Blair said his government suspected that Iran and Lebanon's Hezbollah might be supplying technology and explosives to Shiite Muslim militant groups operating in Iraq, but he provided no proof.

Hezbollah, he said, helps the Iranians as a "proxy ... to do things they didn't want to have to do themselves," Bergner said. He added that Hezbollah did not appear to have an extensive network in Iraq, saying Dakdouk was "being used specifically as a proxy by the Quds Force."

Dakdouk was captured with documents instructing the special groups on techniques, including how to attack a convoy, and a with a personal diary detailing meetings with Iraqi militants. Al-Khazaali also had documents with details on 11 separate attacks on U.S. force, Bergner said.

A total of 18 "higher-level operatives" from the Iranian-backed special groups have been arrested and three others killed since February, Bergner said.