The comments come at a crucial point in talks between Baghdad and Washington over a new security pact that will provide a legal basis for U.S. troops to operate in Iraq when a United Nations mandate expires at the end of the year.
U.S. 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 U.S. combat troops leave the country by October 2010, although Washington has not yet agreed to it, a senior Iraqi official said on Friday.
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 U.S.-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 percent of seats in parliament, has long demanded U.S. 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 Mehdi army, but it's a halfway step to dissolving the Mehdi Army. If the U.S. began to implement a withdrawal timetable we shall complete the path to dissolution," Ubaidi said.
Mehdi Army street commanders in the Sadr City district of Baghdad welcomed the formation of the cultural arm.
"It is a good step to repair our mistakes, especially sectarianism and sectarian killings. We are very sorry about these sectarian killings, because some parties supported this violence for their own advantage," said commander Abu Sadeq.
The U.S. 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."
Iraqi government officials say an agreement is close on a timetable for a U.S. withdrawal. But the White House says it is too soon to say when it can pull out its forces.
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The issue is politically sensitive in the United States ahead of the presidential election in November. Obama has pledged to withdraw all combat troops by mid-2010, while his Republican opponent John McCain refuses to set a date.
The Iraqi proposal would see U.S. forces withdraw from the streets of Iraqi cities by the middle of next year and combat troops return home by October 2010. Some American support units could stay on for another few years, the senior official said.
"As of last night (the schedule) was one of the issues being discussed between the two sides. There is no agreement yet, but this is what the Iraqis are asking for," said the official, who is close to the negotiations.
White House spokeswoman Dana Perino, in Beijing accompanying Bush, said no announcement on an agreement was imminent and it was too early to discuss pullout dates. She cited Bush's earlier statement that any such goals would be "conditions-based."
Iraq's ambassador to the United Nations, Hamid Bayati, said on Thursday an agreement on the U.S. forces' status was close and that the government expected to put it to parliamentarians when they return from summer recess in September.
Sadr's Mehdi Army launched two uprisings against U.S. forces in 2004. He backed Prime Minister Nuri al-Maliki's rise to power in 2006 but split with him last year over the timetable issue.
This year Maliki, a Shi'ite, launched several largely successful crackdowns against militias including the Mehdi Army.
U.S. and Iraqi forces have now deployed in Sadr's Baghdad stronghold, the Sadr City slum, after weeks of heavy fighting in March and April. Prior to the campaign, government influence in among the slum's 2 million people was virtually nil.
Iraqi forces also successfully pushed militias from the centre of the southern city of Basra earlier this year.
Sadr's spokesman said while the "resistance" would not end until U.S. troops left Iraq, the cleric was ready to take positive steps if Washington moved in the right direction.
"If we find (this does not happen) and the U.S. forces change their stance over the timetable, we can change direction also," he said. "This will not mean ending the ceasefire, it will depend on what's going on on the ground."
Additional reporting by Peter Graff, Mohammed Abbas, Waleed Ibrahim, Aws Qusay in Baghdad, Matt Spetalnick in Beijing, Megan Davies in New York and Mariam Karouny in London
Writing by David Clarke, editing by Samia Nakhoul
© 2008 Reuters
Leila Fadel writes about the status-of-forces agreement for McClatchy.