U.S. forces launched a multi-pronged offensive Thursday to drive a stubborn militia from the holiest city in Shiite Islam, as heavy fighting broke out in other cities in southern Iraq and the country's highest-ranking Shiite cleric called for an end to combat.

Shortly before dawn, the U.S. Army's 7th Cavalry Regiment rumbled into Najaf, columns of M1-A1 Abrams tanks and Bradley Fighting Vehicles throwing up an armored cordon around the oldest section of the city. The encircled area extended in a roughly one-mile radius from the shrine of Imam Ali, which militiamen have occupied and used as a base for firing mortars. Grainy gunsight images displayed by Iraqi officials at a Baghdad news conference showed the insurgents' artillery teams inside the shrine complex.

Early Friday, Moqtada Sadr, the radical Shiite cleric who leads the militia, was apparently injured at the shrine during fighting with U.S.-led forces.

Ahmed Shaibani, a Sadr spokesman in Najaf, confirmed that the cleric had been injured in the chest and the leg but said the wounds were not life-threatening.

"If Moqtada Sadr is martyred, there will be no negotiations with the government and there will be a lake of blood in Iraq," Abdul Hadi Darraji, an aide to Sadr, told the al-Arabiya satellite television network. Al-Arabiya reported that Sadr was injured by shrapnel.

Also on Friday morning, U.S. commanders in Najaf reported restrictive new engagement rules in the city and speculated that political negotiation to end the standoff has grown more serious. "There are representatives of the Iraqi federal government in Najaf," Marine Maj. David Holahan said. "What they're doing and who they're talking to I have no idea."

Army Maj. Doug Ollivant said, "We now have very restrictive engagement criteria this morning and there may be some kind of political process going."

On Thursday, Marines raided three buildings adjacent to Sadr's home and launched a nighttime operation elsewhere in the area, accompanied by Iraqi commandos and U.S. Special Forces.

The military offensive appeared to be deliberately paced, unfolding in stages that would allow Iraq's political leadership to calibrate the pressure placed on insurgents who have controlled much of the city for at least the last week. A Marine spokesman, Lt. Col. T.V. Johnson, said combat had been "sporadic, and there have been no major engagements" with the militiamen.

In Baghdad, Interior Minister Falah Naqib said the country's interim government "had no other solution" than to authorize the military operation. Naqib said efforts to broker a lasting peace deal with Sadr and involve his followers in Iraq's interim government had failed.

"Previously, we had tried to solve all our problems politically and peacefully," Naqib said. "What is happening at this stage is not to the benefit of anyone."

Reports from other cities in Iraq's predominantly Shiite south suggested a reprise of an uprising in April, when Sadr's militia, the Mahdi Army, shocked the U.S.-led occupation authority and foreign troops by briefly taking power in several cities.

The fighting Thursday was most intense in Kufa, six miles east of Najaf. U.S. Special Forces called in an AC-130 gunship to destroy a Mahdi Army headquarters and a police station the militia had overrun. At one point, Iraqi security forces crossed the Tigris River in a small boat under machine-gun fire to reclaim a vital bridge, according to the U.S. military.

The Iraqi Health Ministry reported 72 people killed in Kut, 100 miles southeast of Baghdad, according to the Reuters news agency. A commander with the Polish army, which is responsible for security in the area, called the situation serious and held out the possibility of requesting more U.S. troops.

Clashes involving Mahdi Army forces were also reported in Diwaniyah, east of Najaf, and in Nasiriyah, south of Najaf, as well as in Baghdad's Sadr City, the vast Shiite slum named for Sadr's late father, an esteemed ayatollah.

Sadr's supporters marched in the southern port of Basra, and in Amarah several hundred members of the Iraqi National Guard vowed to switch their allegiance to the Mahdi Army until U.S. forces leave Najaf, the Associated Press reported.

The military announced that one U.S. service member was killed in the day's fighting, but did not identify the victim or specify where the incident occurred.

Grand Ayatollah Ali Sistani, who left his Najaf home for the first time in six years just as clashes here began last week, broke his silence on Thursday, calling for an end to all fighting.

"All efforts should be directed to finding a peaceful solution," Sistani said through an aide in London, where he is said to be receiving treatment for a heart condition. "A military solution will resolve nothing."

There were reports of a fresh attempt to renew negotiations between the interim government and Sadr's representatives. But in an interview with the al-Jazeera satellite television network, a spokesman for Sadr dismissed current talks as "superficial."

"Our leader is moving in the direction of martyrdom," said Abdul Hadi Darraji, a sheik. "The government and America want to destroy the Sadr movement. This government wants to fulfill their interests without paying attention to the people's benefit. If they attack the holy shrine of Imam Ali, they will face big rage in Baghdad and other cities."

In Najaf, Shaibani, the Sadr spokesman, said in a telephone interview: "We are ready to face them. We were able to control everything for eight months, and we are able to continue controlling the situation."

U.S. commanders displayed confidence as well, while acknowledging the exceptional tactical challenge they face in trying to dislodge a guerrilla force from a religious site that U.S. troops are under orders not to target.

"They want the Americans to shoot the holy shrine," a Najaf resident advised Ollivant, the Army major, as he mingled with Iraqis in the industrial section of the city. Relatively few in Najaf support Sadr, whom they view as disrespectful of Sistani.

"Take Moqtada Sadr's army out from Najaf," said another resident, Hassan Yusuf, clutching a spark plug box in front of a row of repair shops. "We don't want them."

"Ha! Neither do we!" Ollivant replied.

At the vast cemetery north of the shrine, Ollivant looked in on a company of the Army's 1st Cavalry, 5th Regiment, which reinforced the Marine battalion there when the Mahdi Army ambushed it seven days earlier. That clash quickly escalated into a fight for control of the sprawling necropolis of dun-colored tombs, which has doubled as an arms depot and staging area for the militia.

A week later, the graveyard belonged to no one. The Americans patrolled to deny its use to the militia and draw its fire. A cavalry company called in helicopter fire on insurgents, who fired mortars toward its position earlier in the day.

But the cavalry's Bradley Fighting Vehicles ventured no closer than a mile from the southern border of the cemetery, which ends at the wall surrounding the Imam Ali shrine. Their barrels were pointed toward it, but they were forbidden to fire in that direction for fear of a shot reaching the sacred site and sending repercussions through the Muslim world.

"They're using it as safe haven, and we're being religious about not hitting the shrine. It's like Waco," Ollivant said, referring to the confrontation in the Texas city in 1993 between the FBI and the Branch Davidians, "only they're holed up inside St. Patrick's Cathedral."

The restriction limits the options for U.S. troops. Without a negotiated solution with Sadr, defeating his militia and asserting the authority of the interim government would seem to require an assault on the insurgents sheltered under the golden dome.

Iraqi politicians say the solution is to send homegrown forces into the shrine after U.S. troops fight their way through the warren of alleys and streets surrounding it. U.S. Special Forces have trained a commando force of Iraqis, a unit that fought Sunni Muslim insurgents in Fallujah in April when other Iraqi forces refused.

"The Iraqi police and the Iraqi armed forces will be the forces to liberate the shrine," Naqib said at the news conference.

Defense Minister Hazim Shalan said military operations will continue "until the militias evacuate the holy shrine."

Shalan said Iraqi and U.S. forces had captured about 1,200 people in the Najaf area over the past week. Residents said most of the insurgents were from elsewhere in Iraq, especially Baghdad, but the defense minister said some of the prisoners were foreigners. He provided no additional details, but U.S. commanders in Najaf said the Iraqis had told of capturing prisoners from Syria and Iran.

"The situation taking place is a conspiracy against the Iraqi people," Naqib said. "This is a war that aims to destroy Iraq."

Correspondent Rajiv Chandrasekaran in Baghdad and special correspondent Saad Sarhan in Najaf contributed to this report.

A U.S. soldier guards Iraqi detainees in Shiite Islam's holiest city, Najaf, where U.S. forces encircled an area including the shrine of Ali. Civilians in Najaf flee their homes during the offensive by U.S. forces against militiamen loyal to cleric Moqtada Sadr. An Iraqi pushes a Shiite woman in a wheelchair past a line of U.S. armored vehicles that had entered Najaf early yesterday in a push to expel militiamen.