Iraq's supreme Shiite religious leader, Grand Ayatollah Ali Sistani, demanded Tuesday that all armed forces leave the holy city of Najaf and called on fellow Shiites not to join in a bloody uprising there against U.S. forces. It was his first public effort to end a weeks-old rebellion mounted by the radical cleric Moqtada Sadr.

Sistani was apparently responding to a call to arms issued earlier in the day by Sadr, whose Mahdi Army militia has largely controlled Najaf for weeks. Sistani's words are often heeded by Shiites, although his call Tuesday was not a religious order, or fatwa.

Sadr had invited all Iraqis to come to the southern city and support his uprising, which U.S. troops are struggling to contain. The revolt is one of several serious security issues that U.S. officials face before the scheduled transfer of limited authority to an Iraqi interim government on June 30.

U.S. military options are constricted in part because Najaf is home to one of the most revered sites in Shiite Islam, the Shrine of Imam Ali, and a vast graveyard that is the most favored burial spot among Shiites because of its proximity to the mosque.

For the past month, U.S. officials have been hoping that Sistani would challenge Sadr, whose authority stems largely from his militia, which numbers in the thousands. Sadr has said he would follow a request from Sistani to withdraw from the city, but his rhetoric has grown increasingly militant the longer he has kept U.S. forces at bay.

"So rise up my beloved people," Sadr said in the statement issued by his office in Najaf. He called on "the people of great Iraq to express your opinion" in Najaf "as a reply to the serial violations, in order to be the best people for the best sacred shrines."

Sistani has traditionally shied from political matters. His boldest such overture came last November, when he called for direct elections to establish a post-occupation government, rather than a caucus system favored at the time by U.S. officials.

Tuesday's formal statement, a rare personal message to the public by a man who usually communicates indirectly through aides, came a day after his offices in Najaf were fired on -- by Sadr's men, according to some accounts.

The conflicting statements by Sistani and Sadr appeared to open the way for a test of wills between two clerics with vastly different views of Islam's role in the political future of Iraq. After years of suffering under former president Saddam Hussein's Sunni-led government, Shiites, who make up about 60 percent of Iraq's population, are seeking a representative stake in a post-occupation government.

Shiite communities were once largely receptive to the U.S. invasion as the only viable way to oust Hussein. But the current divisions among Shiites are not only complicating U.S. efforts to establish a broadly acceptable interim government, they are also raising the specter of violence between armed militias loyal to rival Shiite groups.

"We could have a confrontation between Shiite groups in Najaf, and this would be dangerous," said Fatih Kashif Ghitta, a prominent Shiite cleric.

Sadr, who is wanted by U.S. authorities in connection with the killing last year of a rival Shiite cleric, has used his militia, made up largely of disenfranchised young men, to become a major player in Iraq's sectarian politics. Shiite leaders have suggested that Sadr, 31, be given a role in the next government as an incentive for him to demobilize his militia.

On Tuesday, Sadr's forces struck again at U.S. troops, after suffering heavy casualties on Monday. Using mortars and rocket-propelled grenades, his men fired on a small U.S. military camp located between Najaf and Kufa, a town six miles to the northeast that is also a Sadr stronghold. Two U.S. tanks maneuvered toward the camp from a police station in town and were ambushed. There were no reports of casualties on either side.

Sustained fighting in the south has engaged U.S. troops for more than 10 days.

U.S. commanders initially urged patience, hoping to avoid damage to the shrine in Najaf and a pair of shrines in Karbala, a holy Shiite city farther north, where Sadr's militia has also mounted resistance to occupation forces. Rather than confronting U.S. troops directly at the outskirts of the cities, Sadr's men have taken up positions deep inside them and near religious sites. U.S. forces have pursued militia forces, risking damage to the shrines.

So far, Shiite religious leaders who want Sadr removed have complained little about the American tactics. In neighboring Iran, whose population is almost entirely Shiite, Ayatollah Ali Khamenei, the supreme religious leader, this week called on Shiites worldwide to condemn the U.S. offensive across southern Iraq.

U.S. officials had hoped that Shiite leaders could persuade Sadr to abandon his rebellion, disband his militia and give himself up either to U.S. forces or religious leaders and face charges in the killing of Abdel-Majid Khoei, a cleric who was stabbed in April 2003 after returning to Iraq from exile in Britain.

Talks between Sadr and Shiite mediators have broken down over terms that would have put Sadr in their hands rather than in U.S. custody. Without specifying the exact cause, Shiite officials are blaming the United States for the breakdown. "The Americans have added a condition," said Hamid Bayati, a spokesman for the Supreme Council for the Islamic Revolution in Iraq, which wants Sadr out of Najaf.

The continuing violence has exasperated Najaf residents. "From the first day of the crisis, our business stopped. We depend on tourists, and now there are none," said Hadi Basheer, 50, who sells souvenirs.

Ali Hussein, 28, a taxi driver, accused Sadr's militia of harboring common criminals and sympathizers of Hussein. "I want the Americans to solve this, because the Mahdi Army is growing in power," he said.

Special correspondent Saad Sarhan in Najaf contributed to this report.