U.S. forces expanded an offensive against rebel cleric Moqtada Sadr on Sunday by pushing into his stronghold of Kufa for the first time, as his armed followers vanished from the streets of this Shiite holy city.

The battle for southern Iraq, which has occupied U.S. soldiers for weeks, appears to have shifted from a broad engagement across several fronts to a sustained battle aimed at a single elusive objective: Sadr, who leads thousands of militiamen, known as the Mahdi Army.

For seven weeks, U.S. forces have been killing scores of the fighters loyal to Sadr, who has fomented an anti-American insurrection in a region once receptive to the occupation. But the Americans have largely left Sadr alone, fearing that killing him could turn him into a martyr.

The U.S. military's first push into Kufa, where Sadr preaches each Friday, and a strike on a convoy carrying his top aide over the weekend suggest that U.S. commanders have set aside that concern.

U.S. military officers involved in the operation say the assault in Kufa, which began before dawn Sunday and continued into the night, is the latest phase in a campaign that has squeezed Sadr forces out of Kut, Diwaniyah and, over the weekend, the holy city of Karbala.

"We're closing in," said a military official familiar with the operation, declining to characterize it as a hunt specifically for Sadr. "We're keeping the pressure on."

U.S. military officials have five weeks to tame a broad insurgency before an interim Iraqi government assumes limited political authority from the Americans. Quieting the rebellion has become among the most pressing security concerns for U.S. officials as anti-occupation sentiments rise in the run-up to the June 30 handover.

The resistance appeared first in the Sunni Triangle, a region north and west of Baghdad once devoted to ousted president Saddam Hussein. Earlier this month, Marine commanders withdrew from Fallujah, a city in the area and a hotbed of insurgent activity, and ended a siege against the insurgents there. The Marines handed security responsibilities to a group of former Baath Party members who once served in Hussein's army.

The move failed to end attacks against U.S. troops. Two Marines were killed Sunday just outside Fallujah when their convoy was attacked with rocket-propelled grenades. Putting the former army officers in power also angered Iraq's majority Shiite population, which suffered under Hussein's Sunni-led government.

U.S. officials have ruled out negotiating with Sadr to win his withdrawal from Najaf and the demobilization of the Mahdi Army. They have called on Sadr to submit to face charges in the killing of Abdel-Majid Khoei, a moderate rival cleric who was stabbed in April 2003.

Mainstream Shiite political and religious leaders, some of whom command their own party militias, have been unable to agree on who would make up a local security force to control Najaf. Their negotiations have produced new divisions among the Shiites, and U.S. officials have expressed little hope of their success.

"If there is progress to be made, we are open-minded, given that those two conditions are met -- Moqtada al-Sadr faces justice and his illegal militia disbanded and disarmed," said Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq. "But in the interim, we will continue to use our own methods for getting Moqtada's militia off the streets."

Since announcing the arrest warrant against him, U.S. military officials have continued to allow Sadr to travel between Najaf and Kufa to deliver his Friday sermons. But this past Friday, U.S. troops fired on a convoy of cars that resembled Sadr's, hours after he had called on Iraqis to rise up against the occupation during his sermon. U.S. troops captured Mohammed Tabtabaie, a top Sadr aide, and killed his driver in the firefight.

Overnight Saturday, U.S. troops, for the first time, drove deep into Kufa in tanks backed by attack helicopters. U.S. soldiers remained there Sunday, raiding the green-domed Salah mosque. The military said U.S. soldiers killed at least 32 insurgents in Kufa, the Associated Press reported.

Footage from inside the mosque showed blood-red marks curving along the white floor, apparently left by the wounded being dragged to cover. Hospital officials said at least 10 Iraqis died as a result of the fighting.

"It was possible to solve it all peacefully, but the other side refuses," said Mohammad Taqi Modarresi, a cleric and an ally of Sadr in Karbala.

The sudden withdrawal over the weekend of Sadr's forces has perplexed some military officers after weeks of deadly street-to-street fighting. The insurgents, numbering in the hundreds, abandoned their refuge near the sacred shrines of Abbas and Hussein in Karbala. The streets remained calm for the second consecutive day after U.S. forces withdrew from a strategic mosque in the city center. In a time-tested guerrilla tactic, Sadr's forces had vanished.

Bulgarian forces, part of the U.S.-led coalition, manning checkpoints near downtown, said stores opened for business as their owners cleaned up debris left from the weeks of clashes. Some schools also reopened.

"The bad guys may have left . . . we don't know," said a U.S. officer, whose troops were returning to Baghdad after reinforcing units on the outskirts of Karbala.

The pullback of troops from Task Force 1-36, a unit of the 1st Armored Division, came after U.S. officials canceled a push into Karbala set for Friday.

"There was no cease-fire, no deal made in Karbala," said Maj. David Gercken, a spokesman for the 1st Armored Division. "We do not and will not make deals with militias or criminals."

Iraqi police began to patrol the market area around the shrines, where some of the heaviest recent fighting took place, U.S. officials said. U.S. patrols, meanwhile, continued to probe Karbala in nighttime operations and fan out into the countryside looking for arms depots. The searches yielded neither guerrilla suspects nor weapons.

Sadr loyalists warned their U.S. adversaries of the danger of pursuing their leader into Najaf and Karbala and appealed to Shiites to defend the cities.

"The holy cities are of great concern to Shiites and Muslims," said Modarresi, the cleric. He said Sadr was looking for a way to peacefully exit the crisis without surrendering.

Karbala residents, however, seemed eager for Sadr to give up now that U.S. troops had left the city center. Like Najaf, the town depends on religious pilgrims, including many Shiites from Iran, as a major source of income.

"We don't want the Americans here, and they are in their base. It is time also for the others to get out and leave us alone," said Karim Haidar, who sells eggs in Karbala.

Members of an Iraqi family in the town of Kufa look at a damaged house through a shattered window after a U.S. airstrike hit the building.