Iranian forces helped plan one of the most sophisticated militant assaults of the Iraq war — a January raid in which gunmen posed as an American security team and launched an attack that killed five U.S. soldiers, an American general said Monday.

U.S. military spokesman Brig. Gen. Kevin J. Bergner also accused Iran of using its Lebanese ally, the Shiite militia Hezbollah, as a "proxy" to arm Shiite militants in Iraq.

The claims were an escalation in U.S. accusations that Iran is fueling Iraq's violence, which the government in Tehran has denied. It was also the first time the U.S. military has said Hezbollah has a direct role — which, if true, would bring a dangerous new player into Iraq's conflict.

Hezbollah has denied any activities in Iraq, saying it operates only in Lebanon.

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Bergner said a senior Lebanese Hezbollah operative, Ali Mussa Dakdouk, was captured March 20 in southern Iraq. Dakdouk, a 24-year veteran of Hezbollah, was sent to Lebanon "as a surrogate for the Iranian Quds Force" to finance and arm militant cells to carry out attacks on U.S. and Iraqi troops, he said.

The goal was to organize militants "in ways that mirrored how Hezbollah was organized in Lebanon," Bergner said. Hezbollah is one of the region's most disciplined and sophisticated militant groups, able to fight Israel's military to a near standstill in a war last summer.

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 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.

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