BAGHDAD — 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.

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.