After weeks of urban fighting in southern Iraq, U.S. troops suspended attacks on Shiite Muslim insurgents Thursday in response to an offer by rebel cleric Moqtada Sadr to partially withdraw militia forces from the holy city of Najaf and evacuate government buildings.

Sadr's offer, made in writing to Shiite mediators and passed to U.S. occupation authorities, fell far short of the requirements U.S. commanders have said Sadr must meet before they would suspend efforts to subdue the insurgency. During Sadr's seven-week revolt, U.S. officials repeatedly demanded he disarm his Mahdi Army militia and give himself up to Iraqi courts to face charges of murdering a moderate cleric last year.

Nonetheless, U.S. officials said Thursday that once Sadr fulfilled his promises, commanders were prepared to withdraw troops from the city, except those left to guard a few government offices.

"Until that time, coalition forces will suspend offensive operations but will continue to provide security by carrying out presence patrols. Throughout the process, coalition forces will retain the right of self-defense," said Daniel Senor, spokesman for the U.S. occupation authority.

Besides promising to withdraw guerrillas who came to Najaf from elsewhere in Iraq and to evacuate government buildings, Sadr vowed to cease operation of Islamic courts and allow Iraqi police to resume work. He wrote that he would hold "broad talks" with members of the Shiite establishment on the future of the Mahdi Army and his pending court case.

By late Thursday evening, however, it was not clear that even the limited withdrawal of guerrillas was taking place. Reporters observed dozens of gunmen gathered around the Imam Ali mosque, the holiest Shiite shrine in the city. U.S. troops in armored vehicles were positioned not far away at police stations and 1920 Revolution Square.

The day ended on a note that illustrated the multitude of dangers plaguing Iraq. A member of the U.S.-appointed Governing Council, Salama Khufaji, was ambushed while traveling to Baghdad from Najaf, where she had gone Thursday morning to protest the fighting there.

Khufaji survived the attack on her three-vehicle convoy, but four people were killed, including her son. The attack took place near Yusufiya, a town rife with Sunni Muslim and anti-U.S. insurgent groups.

Earlier in the day, Khufaji said in an interview: "The language of military action must be replaced by dialogue. We call on all honorable people to stand on our side to stop the bloodshed."

In Washington, meanwhile, the Pentagon reported that the killings of three Marines in Iraq on Wednesday brought the number of U.S. military dead to more than 800. According to Defense Department figures, 802 U.S. troops have died in Iraq since March 19, 2003, including 587 killed in action.

[On Friday, Japanese media reported that gunmen near Baghdad had attacked a car carrying two Japanese men believed to be journalists, their interpreter and driver, killing one of the four. A government spokesman was unable to confirm details of the attack.]

If the Najaf agreement is carried out and southern Iraq calms down, it would be a major landmark on the road to June 30, the date set for formal transfer of limited authority from the U.S.-led occupation authority to an interim Iraqi government.

Shiite mediators who worked out the deal said it effectively affirmed a broad desire among Shiites for a peaceful transfer of authority that would allow elections to be held in January. Shiite leaders regard the planned vote as a historic chance to rule Iraq, a country that has been dominated by the Sunni minority for centuries.

"We want to safeguard the transformation of Iraq and the lives of civilians," said Adnan Ali, a senior member of the Dawa party, a major Shiite political organization.

Senor, the occupation spokesman, said the Coalition Provisional Authority had not given up its two minimum demands on Sadr.

"We have not altered our position with regard to the need to dissolve and disarm Moqtada's militia throughout Iraq or with Moqtada al-Sadr's obligation to meet the requirements in the arrest warrant issued against him," he said. Senor said he expected Sadr to meet with Shiite mediators "to resolve these issues as soon as possible." He declined to set a deadline for compliance.

However, Mowaffak Rubaie, a Governing Council member and spokesman for the mediators, said the question of Sadr's murder charge "has been left for negotiation." He also raised the possibility that Sadr could participate in party politics. "I see no reason to bar any movement that is willing to resort to the polls," he said.

The negotiations over Sadr marked the second time in recent months that occupation forces, faced with intransigent resistance in an Iraqi city, pulled back from an ultimatum.

In Fallujah, the Sunni stronghold west of Baghdad where Marines fought pitched battles with insurgents, American authorities gave up on demands that fighters surrender their weapons, turn over the killers of four American contractors and surrender foreigners who had joined the insurgency. Instead, they permitted former members of ousted president Saddam Hussein's army to take over security in Fallujah, although ambushes and roadside bomb attacks on the Marines in areas around the city have persisted.

As in Fallujah, the issue in Najaf is whether the compromise will simply turn the city into a haven for anti-American forces. Outside the Imam Ali shrine, Mahdi Army fighters spouted defiance.

"We will not withdraw, because no one in Najaf will protect Moqtada," said Haidar Numani, who traveled to Najaf from Baghdad's Sadr City slum. "I prefer to stay here and defend him."

"We won and they lost," said Hasanein Ali, a guerrilla from Nasiriyah, southeast of Najaf. "They are going to withdraw from the city, not us."

In Baghdad's Sadr City, another Mahdi Army stronghold, residents who oppose Sadr said they were outraged by the news from Najaf.

"They are going to stop going after him after a month when people got killed just to get rid of him? They trust him? If tomorrow he disagrees with them for any reason, he'll call thousands of men into the streets. They should finish what they started," said Salaam Hussein, a teacher who moonlights as a grocer.

"We are the losers, thanks to the Americans," said Nouri Lami, an old man sipping coffee on a cinder block in Sadr City.

The top U.S. military spokesman in Iraq, Brig. Gen. Mark Kimmitt, described Sadr's offer as "a positive step" and said: "The very few forces we had entering any portion of Najaf today, they weren't being attacked, they weren't being shot at. What we're hoping to see in a very short period of time is Iraqi police vehicles going through . . . Iraqi policemen on the corners of the city, Iraqi police buildings back in operation, the governor being able to talk about how to take Najaf forward."

Both Senor and Kimmitt carefully avoided using the word "agreement" to describe the moves by each side. As for the U.S. role, Senor said it was initially passive. A letter from Sadr "was issued," he explained, and "we responded to it. We said we respect the process that has been launched. We are pleased and think this is a positive sign that Iraqis are taking the initiative. . . . And we are going to be responsive."

Senor said that the Sadr letter resulted from negotiations held by a caucus of Shiite leaders on the U.S.-appointed Iraqi Governing Council and that U.S. officials had no contact with Sadr.

Fighting in Najaf had taken a heavy toll on the Mahdi Army, killing scores of guerrillas, Sadr's aides said. Several civilians, including children, were killed in the crossfire, and the city's market areas were heavily damaged.

Special correspondent Saad Sarhan in Najaf contributed to this report.

Iraqis in Najaf demonstrate in support of Shiite cleric Moqtada Sadr, who vowed to withdraw some guerrillas and to evacuate government buildings.