Intense fighting was underway in this city revered by Shiite Muslims and the adjacent town of Kufa Thursday as U.S. and Iraqi forces launched a major offensive designed to crush the insurgent militia of cleric Moqtada Sadr.

During the operation, about 50 U.S. Marines and accompanying Iraqi troops stormed Sadr's house in Najaf and seized it. The rebellious cleric apparently was not found. No shots were heard as troops entered the house and an adjoining school, but minutes later Iraqi police rushed from the house to a nearby hospital, apparently with injured persons.

The Najaf offensive began before dawn Thursday and by late in the day, the interim government's interior minister, Falah Naqib, declared at a Baghdad news conference that about 1,200 individuals and "large amounts of weapons" had been captured without encroaching on the city's shrine of Imam Ali, which is sacred to Shiite Muslims, and has been used as a refuge by the militia.

He said the offensive would continue until Sadr's militia is forced out of its positions in and around the shrine and the vast adjoining Shiite cemetery.

There were no figures available on casualties.

Iraqi and U.S. officials also reported fighting in the Sadr City section of Baghdad, a stronghold of Sadr's Mahdi Army and in the city of Kut, where there were reports of more than 70 Iraqi deaths.

The Mahdi Army has used Najaf, which is about a hundred miles south of Baghdad, as a stronghold since launching a rebellion last April against the U.S. occupation of Iraq. U.S. officials have been reluctant to move against the city in force because of concern about a possible Shiite backlash.

But a week of fierce fighting between U.S. troops and the Mahdi Army in seven cities appears to have changed the thinking of U.S. officials and Iraq's interim Prime Minister Ayad Allawi.

At the news conference in Baghdad, Naqib and Defense Minister Hazim Shalan sought to quell growing criticism of the U.S. and Iraqi military action against the Mahdi Army, casting the joint operation as the last recourse against a foe that threatens national stability.

"We had no other solution," said Naqib, noting that efforts to broker a lasting peace deal with Sadr and involve his followers in the interim government had failed. "Previously, we had tried to solve all out problems politically and peacefully. What is happening at this stage is not to the benefit of anyone."

Seeking to build support among a skeptical population for the offensive, Naqib and Shalan played a 20-second videotape shot from a U.S. military helicopter that they said showed Mahdi Army militiamen firing mortars from within the Imam Ali shrine. The slightly blurry video depicted three men, one of whom appeared to run to and from a launching device, in a courtyard surrounded by large walls. "All of this violates the holiness of the shrines," Shalan said.

Naqib and Shalan disputed the Mahdi Army's claim that they are trying to evict U.S. forces from Iraq. "This doesn't fall under the category of resistance to the occupation," Naqib said. "It is against the Iraqi people. . . . They are trying to derail the rebuilding of Iraq, trying to prevent Iraqis from carrying out their daily life."

They insisted that Iraqi forces were taking the lead role in the military operations and would be the ones to enter the shrine, if necessary. They and a senior U.S. military official said U.S. forces had no plans to enter the holy site.

"The Iraqi police and the Iraqi armed forces will be the forces to liberate the shrine," Naqib said. Shalaan said military operations will continue "until the militias evacuate the holy shrine."

The assault began Thursday, after days of anticipation, as U.S. helicopter gunships and warplanes fired on militia positions in the vicinity of the cemetery in Najaf while separate contingents of U.S. troops moved forward into the city from the north and the south.

Another group, meanwhile, worked to seal off the cemetery and the area around the shrine so that Mahdi Army insurgents would not be able to escape.

Marines officers late in the day said that the raid on Sadr's home was not targeted at the house but instead the three public buildings near his home that had been recruitment and gathering points for his Mahdi Army militia: a maternity hospital and private clinic adjoining the house, and a defunct school across the street.

"These three buildings ever since we've been here have been collection points for the militia," said Marine Maj. David Holahan, executive officer of the 1st Battalion, 4th Marine Regiment. "Sadr's house has nothing to do with this, except for the natural proximity to the buildings.

"We have no idea where Sadr is. He's Prime Minister Allawi's problem."

But Holahan acknowledged that in the course of the raid on the three buildings, Marines did enter Sadr's residence.

During the assault on the three buildings two oil tankers exploded, apparently after taking fire. The dark plumes from the resulting fire filled a section of the sky over the city for most of an hour.

Iraqi police were engaged in combat for Najaf's Revolutionary Square near a police station that has been embattled for the past week by mortar barrages fired from the vicinity of the shrine and the cemetery. Those barrages came fast and furious Thursday morning with at least 25 rounds.

In Baghdad, Brig. Gen. Erv Lessel, deputy director for operations of the U.S.-led military force in Iraq, called the offensive a "clearing operation. . . . The combined Iraqi and multi-national security forces are operating in strict compliance with guidance" from Allawi "to safeguard and prevent possible harm to these holy shrines as well as protect the citizens and future of Iraq," Lessel said.

Holahan said that the old part of Najaf had been "cordoned off with heavy combat."

The mission, Holahan told CNN in a live interview Thursday, "is to eliminate as many members of the militia as rapidly as possible and then come to terms with Moqtada and his militia as soon as possible. So we'll be done as soon as we can be done. Of course the final decision on what we do and where we go will be made by Prime Minister Allawi and the interim government."

Allawi called Thursday on the Shiite militants to put down their weapons and leave the shrine, where they have sought refuge. "These places have never been exposed to such violations in the past," he said, adding that the violence has killed many innocent people.

"Our government calls upon all the armed groups to drop their weapons and return to society. We also call upon all the armed men to evacuate the holy shrine and not to violate its holiness."

On Wednesday, U.S. forces had circulated through Najaf with loudspeakers urging residents to evacuate the city for their own protection and advising members of the Mahdi Army to lay down their arms.

Little international reaction was initially reported Thursday to the Najaf offensive. From London, the Reuters news service said that Iraq's most influential Shiite cleric was urging both the insurgents and U.S. forces to respect the holy city and its shrines.

"Ayatollah Ali Sistani is pained and very sad about what is happening in holy Najaf," Murtada Kashmiri, a Sistani aide, told Reuters. "We call for the holy soil and holy sites of this city to be respected," he added, speaking by telephone. Sistani is undergoing medical treatment in London.

Iran's supreme leader, Ayatollah Ali Khamenei, condemned the military action Wednesday, as U.S. forces readied for combat. "The United States is slaughtering the people of one of the holiest Islamic cities, and the Muslim world and the Iraqi nation will not stand by," Khamenei said in an address broadcast on Iranian state television.

With its overwhelmingly Shiite population and theocratic government, Iran regards itself as the leader of the Shiite world.

"These crimes are a dark blemish which will never be wiped from the face of America. They commit these crimes and shamelessly talk of democracy," Khamenei said. "Shame has no place in their vocabulary."

In other combat Thursday, wire services also reported heavy fighting in the city of Kut located 105 miles southeast of Baghdad. The Reuters news service reported that 72 people were killed and at least 148 wounded Thursday in U.S. air raids and fighting between Iraqi police and Shiite militiamen there.

A U.S. military spokesman said that the fighting began when Iraqi police and national guard units responded on Wednesday to attacks by armed insurgents on Kut's city hall, Iraqi police stations and Iraqi National Guard barracks.

During the resulting firefights, members of both sides were killed and wounded, the military said.

In Sadr City in Baghdad, according to the Iraqi interior minister, U.S. and Iraqi troops attacked "safe houses" where "terrorists and Baathists" have been holed up, defusing more than 300 explosive devices in the process.

Separately, the military announced Thursday that two Marines were killed when their CH-53 helicopter crashed in Anbar Province Wednesday while flying in support of security operations near the Sunni Muslim city of Fallujah.

No enemy fire was observed in the vicinity of the helicopter, the military said.

Fred Barbash reported from Washington. Special correspondent Saad Sarhan contributed to this story from Najaf.