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.

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."

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

Members of the Mahdi Army take cover during an attack on their position in Najaf on Friday. There were also skirmishes to the south, in Karbala.