Anti-U.S. Shiite leader Muqtada al-Sadr ordered most of his militiamen Friday to lay down their arms, and his spokesman said the young cleric might call off all resistance if the Americans accept a timetable to leave Iraq.

In the north, a car bomb exploded Friday evening in a crowded market in the city of Tal Afar, killing at least 21 people and wounding dozens, police and U.S. officials said. Tensions have been rising among ethnic groups throughout the north because of a dispute over control of the oil-rich area around Kirkuk — claimed by Arabs, Turkomen and Kurds.

Moves toward peace from al-Sadr and the bloody attack in Tal Afar, a predominantly Turkoman city with a small Arab community, underscore the complexity of trying to ensure stability here despite a dramatic improvement in security nationwide.

The order from al-Sadr, who mounted two uprisings against U.S.-led forces in 2004, was read to his followers during weekly Friday prayers in Shiite mosques across the country. He instructed his Mahdi Army militiamen to join religious and social welfare classes as part of a new organization — the "Momahidoun," or "those who pave the way."

Posters announcing the Momahidoun have appeared on walls in Sadr City, the cleric's Baghdad base and home to an estimated 2.5 million Shiites.

"It is an ideological, cultural, religious and social army that will be charged with carrying out an intellectual and scientific holy war and to free the minds, hearts and souls from the secularist Western onslaught and is absolutely prohibited from the use of arms," the posters say.

Al-Sadr has been moving toward a primarily political role for his militia since August 2007, when he declared a unilateral cease-fire.

His militia suffered major setbacks in fighting last spring in Sadr City and the southern city of Basra. Cease-fire agreements left the Iraqi army in control of former militia strongholds.

But al-Sadr retains strong support among impoverished Shiites in southern Iraq, and may have concluded that his interests are better served in politics than in armed struggle.

Last June he announced that the Mahdi Army would be transformed into a social welfare movement except for secret special cells that would carry out "resistance" against the Americans and other foreign troops.

But his chief spokesman and a senior Mahdi Army commander both told The Associated Press on Friday that al-Sadr would be willing to stop fighting entirely if the Americans accept a timetable to remove their forces.

"The cells of fighters will stay ready to operate," said Salah al-Obeidi, al-Sadr's chief spokesman. "They will wait and see what is in the security agreement."

For months, U.S. and Iraqi officials have been negotiating security agreements to replace a U.N. mandate that expires at the end of the year. The two sides missed a July 31 target date because of differences over several issues, chief among them immunity for U.S. soldiers.

Two Iraqi officials said Thursday that the Americans had agreed to remove all U.S. troops from Iraqi cities by June 30, 2009 and withdraw combat units by October 2010. All American troops would be gone around 2013, the Iraqis said on condition of anonymity because the talks are ongoing.

But U.S. officials in Washington insisted that no firm dates have been agreed.

Al-Sadr has long opposed the U.S. presence in Iraq, and transforming the Mahdi Army into a social welfare organization would refurbish his image among Shiites who are tired of war. Prime Minister Nouri al-Maliki, a Shiite, has threatened to ban parties that maintain armed militias.

Ali al-Adeeb, a close aide to al-Maliki, described al-Sadr's statement as "a good and a positive step in the right direction."

Despite recent setbacks, the future of the Mahdi Army, estimated between 25,000 and 40,000 fighters, remains a key issue in building long-term security in Iraq. The militia was blamed for much of the horrific sectarian slaughter of 2006 that pushed the country to the brink of all-out civil war.

A recent report by Anthony H. Cordesman of the Washington-based Center for Strategic and International Studies described the Mahdi Army as "the biggest Shiite threat to both the Iraqi government and the U.S.-led coalition."

Even without a U.S. withdrawal agreement, al-Obeidi, the cleric's spokesman, said the special cells would launch attacks only under direct orders from al-Sadr.

"Now our stance is to watch the political developments and the security agreement," he said. "We will see if there will be a withdrawal timetable or not. We will wait for the results. These cells have not yet conducted any operations."

Attacks continue against U.S. troops in Shiite areas, and the U.S. military routinely blames them on "special groups" — Iranian-backed rouge militiamen who have broken with al-Sadr.

In Sadr City, the senior Mahdi Army commander said some of those groups maintain allegiance to al-Sadr while others take orders exclusively from Iran — a charge the Iranians deny. The commander spoke on condition of anonymity because he was not authorized to talk to media.

He acknowledged some opposition within militia ranks to al-Sadr's reorganization plans.

"There has been some dissent among militiamen when the news first broke about the Momahidoun Army," the commander said. "But what Muqtada says always turns out at the end to be good."

He said classes for militiamen will begin in Shiite mosques across the country next month. Teachers will be "religious figures" known for their education and culture, he said.

The commander said the goal was to weed out "criminal elements" and restore the reputation of the militia as a "people's organization."

The bombing in Tal Afar occurred about 6:30 p.m., when the area was packed with shoppers, a senior police official said. The explosion took place about a half hour after expiration of a regular weekly vehicle ban to prevent bombers from targeting weekly Islamic services.

Meanwhile, Georgian President Mikhail Saakashvili told CNN that the country is calling home its troops from Iraq amid heavy fighting in Georgia's breakaway province of South Ossetia.

Georgia has 2,000 troops serving with the coalition forces in Iraq, making it the third-largest contributor after the United States and Britain.

In Washington, a senior defense official said Georgian authorities have asked the United States for help getting their troops out of Iraq. The official, who spoke on condition of anonymity because the discussions have been private, said no formal decision has been made on whether to support the departure, but said it is likely the U.S. will do so.

Also Friday, the U.S. military said two Marines died the day before in a non-combat incident in Anbar province west of Baghdad. No further details were released.