U.S. tanks rumbled Friday into a vast cemetery in the southern city of Najaf, one of Shiite Islam's most sacred places, in pursuit of insurgents loyal to the rebel Shiite cleric Moqtada Sadr. The fighting, which coincided with skirmishes in the other major Shiite holy city, Karbala, demonstrated some of the most aggressive tactics yet employed by U.S. forces against Sadr's Shiite militia.

In images broadcast across the Middle East on Arabic satellite channels, U.S. Army OH-58 Kiowa helicopters fluttered above the ocher and tan necropolis on the edge of the city. Abrams tanks from the 1st Armored Division fired into the warren of tombs. Plumes of gray and black smoke puffed up from between the grave markers, where guerrillas bearing rocket-propelled grenade launchers were positioned.

"The cemetery lost its holiness in the early hours of today when the U.S. forces started to attack," said Khalid Farhan, 55, who owns the Thulfiqar Hotel in downtown Najaf. "Many of the graves have been destroyed. But we can say that people are dying and nice buildings are being destroyed also today. Who cares right now about graves?"

The battle in the cemetery also prompted Sadr's associates outside Najaf to call for a wider mobilization against U.S. forces. Sadr militants rose up in at least one other southern city to seize a government building, a police station and some of its cars.

U.S. officials had hoped a group of mainstream Shiite leaders would persuade Sadr to leave Najaf and demobilize his militia in return for a resolution to his legal problems with the United States. U.S. forces have a warrant to arrest Sadr for his alleged role in the April 2003 slaying of Abdel Majid Khoie, a rival Shiite cleric.

But fighting has overtaken negotiations with Sadr, and the Shiite leadership appears largely incapable of corralling the young cleric. U.S. officials said Sadr and the uprising he has inspired are among the most pressing security problems they must resolve before handing over limited authority to an interim Iraqi government on June 30.

For weeks, Shiite religious leaders have expressed fear that Sadr was endangering Najaf's gold-domed shrine of Imam Ali by using the city center as a sanctuary. Those fears were realized Friday morning when, after clashes in the narrow downtown streets, witnesses said, the dome was pocked with three bullet holes. It was unclear which side had caused the damage or when it had occurred.

"Only Americans have such bullets," said Qais Khazali, a Sadr spokesmen in Najaf, as Mahdi Army fighters draped in head scarves and waving rifles shouted: "They are Jews! They are Jews!"

But Najaf residents, many of whom blame Sadr's militia for ruining the city's economy, said the dome was hit in the confusion of combat.

"If it was done by the Americans, I don't think they did it intentionally," said Ali Awad, a 28-year-old Najaf resident, of the bullet holes. "If they wanted to destroy the shrine, they could destroy it. But they don't."

In Baghdad, American officials said it was unlikely that U.S. forces had hit the dome, because they were firing in the opposite direction, toward guerrillas in the cemetery. Believed to be the second-largest cemetery in the world, the Wadi al-Salam is roughly a mile from the Shrine of Imam Ali. The name means Valley of Peace.

U.S. military officers characterized the push into Najaf as a reaction to mortar attacks on two police stations, not as a new offensive to drive out Shiite insurgents who only recently have taken up positions deep in the city.

[On Saturday, the U.S. military announced the deaths of four soldiers in separate incidents Friday, according to an Associated Press report. A soldier died from wounds received during a mortar attack, and another soldier was killed by a sniper. Earlier, a military vehicle overturned during a patrol, killing a soldier. All three soldiers belonged to the U.S. Army's 1st Armored Division and all died south of Baghdad, the Associated Press reported.

The fourth soldier died of natural causes in the headquarters of the U.S.-led coalition in Baghdad, the report quoted the military as saying.]

Until Friday, U.S. forces had been content to chip away at Sadr's forces on the outskirts of Najaf, fearing that a frontal attack near the holy places would inflame Shiite opinion. Shiites, who account for 60 percent of Iraq's population, have largely accepted the U.S.-led occupation after years of repression under the former government of Saddam Hussein, which was dominated by Sunni Muslims.

U.S. military officials also have been reluctant to move against Sadr personally for fear of angering his followers. The operations on the outskirts of Najaf and other southern cities were meant to press him to accept a negotiated solution.

U.S. military officials said Sadr, whose fighters are mostly from outside Najaf, is widely unpopular inside the city. Najaf's primary industry is catering to Iranian Shiite pilgrims, a trade that blossomed after Hussein's ouster but has dwindled to nothing with the violence.

On Thursday, Sadr's militants broke up public demonstrations against him by firing rifles into the air. Shiite leaders called off an anti-Sadr rally scheduled in Najaf.

U.S. military officials said that while they believe Sadr must be defeated now to prevent his influence from spreading, they are still constrained by concerns about damaging the holy sites.

"We want to do everything we can to avoid widening this problem from Moqtada to something more," said Brig. Gen. Mark Kimmitt, chief spokesman for the U.S. military in Iraq. "We certainly want to avoid being drawn into an attack that would create an incident that has strategic impact."

The images of U.S. tanks near the Shrine of Imam Ali, one of Shiite Islam's holiest mosques, brought calls for resistance during Friday prayer services at pro-Sadr mosques.

"Where are those people who said Najaf is a red line?" Abdul Hadi Daraji asked several thousand worshippers at the al-Hikma Mosque in Sadr City, an eastern Baghdad slum named for the young cleric's father, who was assassinated in 1999. "I'm asking all the people here, 'If anyone feels he is able to go to Najaf to support your brothers, go!' "

Witnesses said Najaf's Thulfiqar Hotel came under fire Friday morning as U.S. tanks rattled through the streets. Correspondents from the Reuters news agency, Agence France-Presse and the Associated Press, as well as The Washington Post and the U.S.-funded al-Hurra satellite channel, reside at the hotel.

As described by witnesses, tank rounds struck the roof, the lobby and a courtyard behind the building, sending cameras toppling and reporters ducking for cover. Some suffered minor injuries.

"They first made warning shots," said Farhan, the owner. "When the reporters wouldn't move [from the roof] they shot."

In sporadic clashes throughout the day, U.S. forces killed at least four Iraqis, including two Mahdi Army fighters, and wounded 25 others, according to hospital reports. And Ahmed Ali, assistant director of Najaf's Hakim Hospital, said that "the number of killed could increase because we have some critically wounded people." Not all the dead reach hospitals, Najaf residents said; some are immediately buried by their comrades.

Despite the intense fighting, Sadr delivered a scheduled sermon at Friday prayers in the main mosque in Kufa, roughly six miles east of Najaf. He warned that others were trying to divide the Shiite community and advised a rival Shiite militia attached to a political party not to fall for the ploy.

Over the past three days, U.S. forces have also fought pitched gun battles with insurgents near the shrines of Hussein and Abbas in Karbala -- mosques that the insurgents have sought to use as fortresses. U.S. officials estimated that more than two dozen insurgents have been killed in Karbala in recent days.

Combat continued there Friday, and persistent firefights made gathering the dead and wounded Iraqis impossible, witnesses said. Those who fell near houses were quickly pulled inside and later returned to their families.

For the first time, fighting spread to Nasiriyah, a Shiite city southeast of Najaf. Insurgents overran the governor's office, a police station and a hotel. At the police station, the militants briefly held 16 U.S.-trained Iraqi policemen and seized four patrol cars, witnesses there said.

The action followed a call by the Mahdi Army commander in Nasiriyah, which has been largely calm in recent weeks, to rise up. Kimmitt said during his regular afternoon news conference that the situation there was under control. A few hours later, however, Sadr's militiamen attacked the occupation authority offices. One Philippine contractor was wounded.

Special correspondents Omar Fekeiki and Naseer Nouri in Baghdad, and Saad Sarhan in Najaf contributed to this story.

Smoke rises from the Wadi al-Salam cemetery in the Shiite city Najaf after U.S. forces attacked guerrillas loyal to cleric Moqtada Sadr. Member of Moqtada Sadr's militia runs through the cemetery. U.S. forces stepped up attacks on the Shiite militia after being unable to persuade its leader to leave Najaf.Shiite militiamen celebrate as they display destroyed U.S. Army equipment in Najaf after the American attack on forces of rebel cleric Moqtada Sadr.Members of Mahdi Army take cover during attack on their positions in Najaf. There were also skirmishes in the holy city of Karbala to south. U.S. officials say the uprising inspired by Iraqi Shiite cleric Moqtada Sadr poses deep security problems.