By Dean Yates and Sean Maguire

BAGHDAD, Feb 11 (Reuters) - U.S. forces in Iraq have been informed influential Shi'ite cleric Moqtada al-Sadr will extend a ceasefire order to his feared Mehdi Army militia when it ends this month, the U.S. military commander in Iraq said on Monday.

Sadr, whose followers hold sway in poor Shi'ite areas of Baghdad and across parts of the south, imposed the six-month truce on Aug. 29 after deadly clashes between the Mehdi Army and police in the holy city of Kerbala.

Iraqi and U.S. officials say his order helped cut violence in Iraq and a return to fighting could jeopardize those gains.

"What we have heard is that the ceasefire will continue ... that's what we have been told," General David Petraeus said in an interview, without saying who had passed on the information.

"I don't know the duration of (the extention). But the informed rumours that have come to us are that it would be extended. But we are truly in the wait-and-see mode."

Sadr, the son of a revered Shi'ite cleric killed under Saddam Hussein, led two uprisings against U.S. forces in 2004.

He ordered the truce to reorganise the splintered militia, which was acccused by Washington and Iraqi politicians of involvement in sectarian violence in Iraq after suspected al Qaeda militants bombed a Shi'ite shrine in Samarra two years ago.

Sadr ordered the Mehdi Army last week to maintain the truce for now after militiamen fought security forces in Baghdad. His spokesman, Salah al-Ubaidi, said the truce should be observed until militiamen were told otherwise.

Petraeus said he would wait to see what Sadr did and the military was preparing for whatever the decision was.

"For the good of Iraq and to enable us to continue pursuing al Qaeda without watching our back, to enable Iraq to continue momentum in the areas of rebuilding and reconstruction ... all of that argues for a continuation of the ceasefire," he said.

"The ceasefire has been ... a factor that has resulted in a reduction of violence in Iraq."

The Pentagon once described the Mehdi Army, which has tens of thousands of members, as the greatest threat to peace in Iraq. It now reserves that term for Sunni Islamist al Qaeda.

"QUIT SHOOTING AT US"

Sadr, a reclusive cleric in his 30s, has long demanded U.S. forces leave Iraq. In April last year he pulled six ministers from his political movement out of the government after Prime Minister Nuri al-Maliki refused to set a timetable for the withdrawal of American forces.

Petraeus said that if Sadr wanted to see U.S. forces leave, it made sense to renew the ceasefire.

"Frankly, if the overall objective is to get a timetable for the removal of coalition forces or something like along those lines, quit shooting at us because over time we are going to go," Petraeus said.

Statements from Sadr's camp have indicated growing unhappiness that followers were being targeted by Iraqi forces.

The U.S. military says it has also continued to target what is calls "rogue" Mehdi Army militants, who U.S. commanders say receive funding and weapons from neighbouring Iran.

Ubaidi has said Sadr has been gauging the mood of senior figures before deciding whether to extend the truce. He has said Sadr will issue a statement around Feb. 23 if he agrees to renew the ceasefire and silence would mean it was over.

The International Crisis Group think tank said last week the respite offered by the truce was "exceedingly frail".

(Editing by Samia Nakhoul)

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