FALLUJAH, Iraq - Iraqi security forces took over positions from withdrawing U.S. Marines today, and a U.S. official said an agreement had been reached to allow an Iraqi security force to patrol the city and end the monthlong siege.

Members of the 1,100-member force moved into the former Marine positions in southeastern Fallujah and raised the Iraqi flag.

Negotiations were also taking place in the southern city of Najaf, where tribal leaders and police discussed a proposal to end a standoff between soldiers and militiamen loyal to radical Shiite cleric Muqtada al-Sadr. In a sermon, al-Sadr remained defiant, saying he rejected "any appeasement with the occupation."

The commander of the new force is Maj. Gen. Jassim Mohammed Saleh, a veteran of Saddam's Republican Guard. He shook hands with Marine commanders at a post on the southeastern entry to the city.

A senior defense official at the Pentagon said the Iraqi soldiers' initial mission is to man checkpoints around the city. Marines will remain on or near the city's perimeter and plan at a later stage to conduct their own patrols inside the city, the official said on condition of anonymity.

The Iraqi force will be all volunteer and will consist of former Iraqi soldiers from the Fallujah area who are vetted by U.S. authorities, the official said.

Marines went into Fallujah to find those responsible for the March 31 killing and mutilation of four American contract workers, whose bodies were burned and dragged through the streets.

However, the United States has been under intense pressure from the United Nations, its international partners and its Iraqi allies to end the bloodshed, in which hundreds of Iraqi civilians are believed to have died.

Saleh is a veteran of Saddam's Republican Guard. He met with tribal leaders in a mosque on this morning, wearing his uniform from the former Iraqi military with his general's insignia.

"Fallujah residents have chosen Maj. Gen. Jassim Mohammed Saleh to form and lead a unit that will be in charge of protecting the city," said Iraqi Brig. Gen. Shakir al-Janabi, who expects to be part of the new force. "Our force will handle the security issue today in cooperation with Iraqi police."

One of three battalions of U.S. Marines packed up and withdrew from most of its positions in an industrial zone in the southern area of the city. U.S. military guards permitted civilian cars to enter the city after undergoing searches.

In an apparent move to speed the Fallujah agreement, U.S. authorities Thursday released the imam of the city's main mosque, Sheik Jamal Shaker Nazzal, an outspoken opponent of the U.S. occupation who was arrested in October.

Skirmishes had continued between Marines and guerrillas during the negotiations.

Three F/A-18 Hornets flying off the aircraft carrier USS George Washington in the Gulf dropped three 500-pound bombs Thursday on targets in the Fallujah area, Navy spokesman Lt. Cmdr. Danny Hernandez said.

Witnesses reported rockets fired into the Golan neighborhood, a bastion of the insurgency, and two houses were on fire. Marines and guerrillas have clashed repeatedly in the northern district since Monday.

U.S. Marines encircled the city of 200,000 on April 5. Hospital officials said more than 600 Iraqis, many of them civilians, were killed in the fighting along with eight U.S. Marines. But the figures were disputed by Iraq's health ministry and an exact toll was not known.

Ten U.S. soldiers and a South African civilian were killed in attacks elsewhere in Iraq on Thursday, including eight Americans who died when a bomb hit as they tried to clear explosives from a road south of Baghdad.

The American deaths Thursday raised to 126 the number of U.S. troops killed in combat in April, the bloodiest month for U.S. forces in Iraq. The military said another soldier died in a vehicle accident in western Baghdad.

At least 736 U.S. troops have died in Iraq since the war began in March 2003. Up to 1,200 Iraqis also have been killed this month.

Negotiations continued in to end the standoff with militiamen loyal to al-Sadr.

Ahmed Shaybani, a spokesman for al-Sadr, told The Associated Press that talks were under way between Najaf police and tribal leaders. He said a proposal emerged under which al-Sadr followers would hand over security to the Najaf police and Sadr's Mahdi army would leave the city.

Shaybani said the proposal would be accepted if the Americans agreed not to enter Najaf and did not act in a hostile way toward its holy sites. Al-Sadr would remain in the city.

In a sermon at a mosque in nearby Kufa, al-Sadr remained defiant.

"Some people have asked me to tone down my words and to avoid escalation with the Americans," al-Sadr said. "My response is that I reject any appeasement with the occupation and I will not give up defending the rights of the believers. America is the enemy of Islam and Muslims and jihad is the path of my ancestors."

Lt. Col. Pat White said U.S. forces were holding back to give talks a chance and out of respect for Friday, the Islamic day of prayer.

"We want to show that we respect what that day means to the Islamic world," White said, adding that U.S. forces will closely monitor the speeches that clerics give at prayer services.

In other developments:

A member of the city council of Baqouba, north of Baghdad, was wounded in a drive-by shooting, said Baqouba hospital official Hussein Ali.

Iraqi security officers detained four Afghans 15 miles south of the northern oil city of Kirkuk, Gen. Anwar Amin, head of Iraqi security forces said today. The Afghans, who were detained Thursday, had no identity documents with them, Amin said.