Iraqi Shia cleric Moqtada al-Sadr would dissolve his Mahdi army militia if the United States started withdrawing troops according to a set timetable, a spokesman said.
A car bomb in the northern town of Tal Afar killed 18 people and wounded 25, police said, an attack that demonstrated the potential for violence that persists in a country that has become far more peaceful over the past year.
Police said the bomb struck a crowded vegetable market. The town is near the city of Mosul, in an area where US-backed Iraqi forces have launched a crackdown on al-Qaeda Sunni Arab militants in recent months.
Sadr's decision to link disarming his militia to a US withdrawal comes at a crucial point in talks between Baghdad and Washington over a security pact to provide a legal basis for US troops in Iraq when a UN mandate expires at the end of the year.
US president George W. Bush has refused to set a firm timetable for withdrawing 144,000 American troops from Iraq, but spoke last month of a general "time horizon" for a pullout.
Iraqi negotiators have proposed that US combat troops leave the country by October 2010, although Washington has not yet agreed to it, a senior Iraqi official said today.
If agreed, the timetable would mean the Bush administration effectively adopting a schedule very close to that proposed by Democratic presidential candidate Barack Obama, who opposed the 2003 US-led invasion of Iraq.
A ceasefire imposed by Sadr on his militia a year ago has been a major factor in a drop in violence to four-year lows. Sadr, whose political movement controls 10 per cent of seats in parliament, has long demanded US troops leave Iraq.
"We feel there's a serious intention by the American forces for a withdrawal timetable at the very least," Sadr's spokesman Salah al-Ubaidi said before Friday prayers, when the cleric launched a new cultural wing of his movement.
"It should not be considered an end to the Mahdi army, but it's a halfway step to dissolving the Mahdi Army. If the US began to implement a withdrawal timetable we shall complete the path to dissolution," he said.
The US embassy in Baghdad said in a statement that all illegal armed elements in Iraq must disband to put an end to violence and it called on Sadr "to renounce violence and participate peacefully in the Iraqi political process".