The general also said that Dakdouk was a liaison between the Iranians and a breakaway Shiite group led by Qais al-Khazaali, a former spokesman for cleric Muqtada al-Sadr. Bergner said al-Kazaali's group carried out the January attack against a provincial government building in Karbala and that the Iranians assisted in preparations. Al-Khazaali and his brother Ali al-Khazaali were captured with Dakdouk.

Dakdouk told U.S. interrogators that the Karbala attackers "could not have conducted this complex operation without the support and direction of the Quds force," Bergner said.

Documents captured with al-Khazaali showed that the Quds Force had developed detailed information on the U.S. position at the government building, "regarding our soldiers' activities, shift changes and defenses, and this information was shared with the attackers," Bergner said.

The Karbala attack was one of the boldest and most sophisticated against U.S. forces in four years of fighting in Iraq, and U.S. officials at the time suggested Iran may have had a role in it.

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.