NAJAF, Iraq - A 2,500-strong U.S. force, backed by tanks and artillery, pushed to the outskirts of the Shiite holy city of Najaf today for a showdown with a radical cleric. Near Fallujah, a U.S. military helicopter crashed, but there was no indication anyone aboard was hurt, a Marine commander said.

An insurgent said he shot the chopper down with a rocket-propelled grenade, although Marine Lt. Col. Brennan Byrne said it was not known what caused the Sikorsky H-53 to crash. U.S. troops blew up the downed craft to keep it from being looted, Byrne said.

An Associated Press reporter saw it burning 12 miles east of Fallujah in the village of Zawbaa. U.S. troops trying to reach the downed aircraft were attacked by gunmen, the reporter said. Witnesses said four U.S. soldiers were shot by insurgents.

On Monday, meanwhile, Gen. John Abizaid, the top commander of U.S. forces in the Middle East, said he has asked Defense Secretary Donald Rumsfeld to adjust the U.S. troop rotation into and out of Iraq this spring so that U.S. commanders can have the use of perhaps 10,000 more soldiers than they otherwise would have.

On the way to Najaf, the U.S. force's 80-vehicle convoy was ambushed Monday night by gunmen firing small arms and setting off roadside bombs north of the city. One soldier was killed and an American civilian contractor was wounded, officers in the convoy said.

The top U.S. commander in Iraq, Lt. Gen. Ricardo Sanchez, said their mission was to "capture or kill" radical Shiite cleric Muqtada al-Sadr.

Units set up a cordon on approaches to the city, barring militiamen from leaving.

Iraqi leaders launched hurried negotiations aimed at averting a U.S. assault on the city, site of the holiest Shiite site, the Imam Ali Shrine.

The sons of Iraq's three grand ayatollahs - including the most powerful one, Ali al-Husseini al-Sistani - met al-Sadr Monday night in his Najaf office and assured him of their opposition to any U.S. strike.

"They agreed not to allow any hostile act against Sayyed Moqtada al-Sadr and the city of Najaf," said a person at the meeting, speaking on condition of anonymity.

The delegation also was reportedly trying to work out a compromise to prevent a U.S. attack.

Col. Dana J.H. Pittard, the commander of the force, said his troops were aware that a "single shot in Najaf" by U.S. soldiers could outrage Iraq's powerful Shiite majority.

The grand ayatollahs - older, moderate leaders with immense influence among Shiites - have long kept the young, fiercely anti-American al-Sadr at arm's length. The dispatch of the delegation reflected the eagerness to avoid bloodshed in Najaf and the new influence that the uprising by the al-Mahdi Army's militia has brought al-Sadr.

In a concession to American demands, al-Sadr ordered his militiamen out of police stations and government buildings in Najaf and the nearby cities of Karbala and Kufa. Police were back in their stations and on patrols, while al-Sadr black-garbed gunmen largely stayed out of sight.

But the militia rebuffed a U.S. demand to disband.

Earlier today, al-Sadr militiamen based in the main mosque in the nearby city of Kufa opened fire on a passing patrol of Spanish forces, prompting a short gunbattle.

Overnight, a mortar was fired at the Spanish base between Kufa and Najaf, and Spanish forces repelled an attack on a nearby water distillation plant.

While a cease-fire has kept Fallujah relatively calm for four days, the area between the besieged city and Baghdad has seen heavy clashes by insurgents and U.S. forces. An Apache helicopter was shot down Sunday in nearby Abu Ghraib, killing its two crewmembers.

Before todayTuesday's helicopter crash, a U.S. convoy was attacked near the same site, and two Humvees and a truck were burning, said witnesses, who also reported U.S. casualties.