BAGHDAD — Shiite cleric Muqtada al-Sadr ordered most of his militiamen to disarm but said Friday he will maintain elite fighting units to resist the Americans if a timetable for the withdrawal of U.S. troops is not established.

Fighters in the Sunni-led insurgency, meanwhile, set off a car bomb at a market in the northern city of Tal Afar, killing 21 people and wounding dozens, Iraqi police said. It was the latest in a series of deadly attacks seeking to chip away at recent security gains.

Al-Sadr's statement — read to worshippers during Friday prayers in Baghdad's former militia stronghold of Sadr City — was in line with details revealed earlier this week and appeared to be an extension of plans he announced in June aimed at asserting more control over the militia.

"Weapons are to be exclusively in the hands of one group, the resistance group," while another group called Momahidoun is to focus on social, religious and community work, Sadrist cleric Mudhafar al-Moussawi said.

He said the announcement was particularly aimed at members of al-Sadr's Mahdi Army militia, which has been blamed for some of the worst violence against American troops and rival Sunni Arabs.

Thousands of worshippers streamed out into the streets after the Islamic service, burning an American flag and shouting: "No, no to America. No, no to occupation."

The cleric has linked the reorganization of the Mahdi Army to U.S.-Iraqi negotiations over a long-term agreement that would extend the American presence in Iraq after a U.N. mandate expires at the end of the year. Al-Sadr and his followers want the deal to include a timeframe for an American withdrawal and have warned they may not suspend operations without such a clause.

Several cease-fires by al-Sadr have been key to a sharp decline in violence over the past year, along with a Sunni revolt against al-Qaida in Iraq and a U.S. troop buildup. But American officials still consider his militiamen a threat and have backed the Iraqi military in operations to try to oust them from their power bases in Baghdad and elsewhere in Iraq.

The fighting cells will be "small and limited" and will only launch attacks under direct orders from al-Sadr in case of "dire necessity," the cleric's spokesman, Sheik Salah al-Obeidi, told The Associated Press in the holy city of Najaf.

He also ruled out attacks on Iraqis.

"Now our stance is to watch the political developments and the security agreement. 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," he added.

Two Iraqi officials close to Prime Minister Nouri al-Maliki have said government and U.S. negotiators are near an agreement on all American combat troops leaving Iraq by October 2010, with the last soldiers out three years after that. The officials all spoke on condition of anonymity because the talks were still under way.

U.S. officials, however, insisted no dates had been agreed.

"It's premature to say what the aspiration goals and time horizons are going to be," and a date for troop withdrawals will not be "plucked out of thin air," White House press secretary Dana Perino said, speaking to reporters in Beijing on Friday where President Bush is attending the Olympics.

Throughout the conflict, Bush steadfastly refused to accept any timetable for bringing U.S. troops home. Last month, however, Bush and al-Maliki agreed to set a "general time horizon" for ending the U.S. mission.

The car bomb in Tal Afar exploded by a food market about 6:30 p.m., when the area was crowded with shoppers, police said. One official said Iraqi soldiers had searched the car at a checkpoint leading to the market but failed to notice the explosives.

Two local officials, who spoke on condition of anonymity because they weren't authorized to release the information, gave the casualty toll as 21 dead and 72 wounded. The U.S. military confirmed the attack but said initial reports indicated 15 people were killed and 50 wounded.

"I was standing near my cart when I heard a big explosion and I felt as if hell was in front of me," said Hussein Ali, a 15-year-old food vendor wounded in the head and legs. "The next thing I knew I was in the hospital receiving treatment," he said from his bed.

Tal Afar, a predominantly Shiite Turkomen city 260 miles northwest of Baghdad, also was hit by a car bombing July 16 that killed at least 18 people, including seven children.

U.S.-Iraqi military operations are currently under way pursuing al-Qaida in Iraq fighters and other insurgents in Mosul and elsewhere in the north.

Ethnic tensions also have been rising between Turkomen, Arabs and Kurds in that region over the status of the oil city of Kirkuk.

Kurdish leader Massoud al-Barzani visited Kirkuk on Friday and called for the rival factions "to have an open dialogue" to resolve their disagreement over sharing control of the city.

His appeal came two days after the dispute blocked passage of a provincial elections law, casting doubt on whether U.S.-backed balloting can be held this year in Iraq's 18 provinces. The bill failed because the sides were unable to come to agree on a power-sharing deal for the region around Kirkuk, the center of Iraq's northern oil fields.

Kurds consider Kirkuk their ancestral capital and want to incorporate it into their self-ruled region in the north. Most Arabs and Turkomen want Kirkuk to remain under central government control.

In Washington, the State Department expressed irritation that the parliament had gone into summer recess without having reached a compromise on the matter.

"The status of Kirkuk is indeed a sensitive issue that needs to be addressed in a serious fashion, but it is an issue that cannot be solved through the legislative mechanism of the election law," spokesman Gonzalo Gallegos said. "The election law should not be held hostage to that problem."

Associated Press writer Saad Abdul-Kadir contributed to this report.