NAJAF, Iraq - Thousands of U.S. troops and Iraqi soldiers launched a major assault today on militiamen loyal to a radical Shiite cleric in Najaf, with explosions and gunfire echoing around the holy city's revered Imam Ali shrine and its vast cemetery.

The coalition forces were trying to crush an uprising led by cleric Muqtada al-Sadr, whose fighters have been battling U.S. troops in Shiite strongholds across Iraq for a week. Hundreds of people have fled in the last few days, moving in with relatives and friends in quieter neighborhoods, or out of Najaf entirely.

"Major operations to destroy the militia have begun," said U.S. Marine Maj. David Holahan, executive officer of the 1st Battalion, 4th Marines Regiment.

A column of U.S. tanks lined one edge of the cemetery, as a helicopter flew overhead on patrol. Soldiers crawled on the roofs of buildings, setting up positions.

Two soldiers were injured when hit by a mortar shell, the military said.

"It's pretty standard, they'd push up here, fire off a few rounds, fire RPGs, then leave," said Capt. Patrick McFall.

The offensive risked enraging Iraq's Shiite majority - including those who do not support the uprising - if it targets the shrine, where many of the insurgents have taken refuge. The shrine, the cemetery, and Najaf's Old City were cordoned off and any attack on them would be led by Iraqis - some with minimal training - to deflect anger.

U.S. Brig. Gen. Erv Lessel, the deputy director for operations, suggested there were no immediate plans to attack the mosque.

"Today's operations are designed to restrict freedom of movement of Sadr forces in (nearby) Kufa and Najaf and to further isolate them in these mosques which they use as a base of operations," Lessel said.

U.S. commanders say interim Iraqi Prime Minister Ayad Allawi would have to approve any operation at the shrine itself, and operation that involves entering the shrine would likely involve Iraqi national guard troops, not U.S. forces.

"The combined Iraqi and multinational security forces are operating in strict compliance with guidance from the prime minister to safeguard and prevent possible harm to these holy shrines as well as protect the citizens and future of Iraq," he said.

Also today, officials said attacks by insurgents and Mahdi militants on government buildings and police stations in the southern city of Kut have killed at least 70 people, all of them Iraqi.

Iraqi forces fought off the militants who targeted the city hall, police stations and Iraqi National Guard barracks, the U.S. military said, causing casualties on both sides.

"Seventy-two people were killed and 148 injured in clashes in the last 24 hours," said Falah al-Bairaman, director-general of health for Wasit, the province of which Kut is the capital city.

On Wednesday, Al-Sadr, the firebrand cleric leading the insurgents, exhorted his followers to fight on even if he is killed.

The United States had announced its plan for the offensive Wednesday, and in response, al-Sadr loyalists in the southern city of Basra threatened to blow up the oil pipelines and port infrastructure there. A similar threat Monday caused oil officials to briefly stop pumping from the southern oil wells.

Elsewhere, two U.S. Marines were killed when a CH-53 helicopter crashed landed in the volatile Anbar province west of Baghdad, the military said Thursday. Three other people were injured in the crash Wednesday night. The military said that no enemy fire was observed at the time.

An Islamic Web site carried a videotape Wednesday that appeared to show militants in Iraq beheading a man they identified as a CIA agent. The authenticity of the videotape could not be verified immediately. A U.S. official, speaking on condition of anonymity, said CIA officials have accounted for all employees and no one is missing.