Shi'ite cleric Moqtada al-Sadr threatened on Tuesday to end a truce he imposed on his militia last year, raising the prospect of worsening violence just hours before top U.S. officials testified on Iraq in Washington.

The U.S. commander in Iraq, General David Petraeus, told a Senate panel he was recommending a 45-day pause in troop cuts after July when an initial drawdown is completed. He said he would then decide over time when to recommend more reductions.

His comments suggest Washington will still have some 140,000 troops in Iraq in the run-up to the U.S. presidential election in November, roughly the same number as before reinforcements were sent in early 2007 to halt Iraq's slide into civil war.

The Mehdi Army militia ceasefire had been credited with helping to sharply reduce violence across Iraq. Scrapping the truce could trigger widespread fighting with security forces and plunge Iraq back into a deadly spiral of sectarian violence.

Sadr's warning came a day after Shi'ite Prime Minister Nuri al-Maliki threatened to bar the cleric's movement from political life unless he disbanded the militia, which has fought fresh battles with Iraqi and U.S. forces in the past two weeks.

"The Iraqi government should know that the Mehdi Army will stand shoulder to shoulder with the Iraqi people to provide all they need from security, stability and independence," Sadr said in a statement sent to his offices across Iraq.

"If it is required to lift the freeze (ceasefire) in order to carry out our goals, objectives, doctrines and religious principles and patriotism, we will do that later."

Hours later the government imposed a one-day curfew on the capital for Wednesday - the fifth anniversary of the fall of Baghdad - but gave no reason. Sadr had called for a big anti-U.S. march in Baghdad on Wednesday but cancelled it for fear his followers would be attacked.

Sadr urged his Mehdi Army to "continue your jihad and resistance" against U.S. forces, although he did not spell out if this was an explicit call for attacks on American soldiers.

Despite the more than 7-month-old ceasefire, Sadr's followers have clashed with Iraqi troops and U.S. forces in the south of the country and Baghdad in the past two weeks in the country's worst violence since the first half of 2007.

FRESH FIGHTING

American and Iraqi troops clashed with gunmen in the cleric's Baghdad stronghold of Sadr City for a third day running on Tuesday, part of an upsurge in fighting in the capital.

Hospital officials in the slum said 13 people had been killed and 140 wounded on Tuesday.

An Iraqi cameraman working for Reuters in Sadr City was lightly wounded by shrapnel in what witnesses said was a U.S. air strike in the slum.

Lieutenant-Colonel Steven Stover, a U.S. military spokesman, said four Hellfire missiles were fired at gunmen and mortar crews in Sadr City in three separate air strikes.

In Washington, Petraeus said security gains in Iraq were "fragile and reversible".

Petraeus said after five extra combat brigades had left Iraq by July he would "undertake a 45-day period of consolidation and evaluation". But he declined to estimate how many U.S. troops would be in Iraq at the end of 2008.

Iraq's government had no objection to the pause, spokesman Ali al-Dabbagh said.

"The Iraqi government thinks Petraeus's decision is correct," he told Reuters.

The United States is withdrawing about 20,000 combat troops from Iraq through July. It has 160,000 troops in Iraq now.

Sadr's aides had said on Monday the cleric was ready to disband his militia if religious leaders demanded it. Sadr has a history of pulling back unexpectedly from confrontation and then allowing his militia to show its strength.

Maliki ordered a crackdown on the militia two weeks ago in the southern city of Basra, provoking widespread clashes.

That fighting ebbed just over a week ago when Sadr ordered the militia off the streets, but picked up again on Sunday with clashes around Sadr City.