FACTBOX: What is the Mehdi Army?

(Reuters) - Iraqi Shi'ite cleric Moqtada al-Sadr is ready to disband his Mehdi Army militia if Shi'ite religious leaders he plans to consult order him to do so, a senior aide to Sadr said on Monday.

* Formed after Saddam Hussein's overthrow in April 2003, the Mehdi Army is loyal to Sadr, a fiercely outspoken cleric who is popular among Iraq's poor, urban Shi'ite majority.

* Sadr led rebellions against U.S.-led forces in 2004. In August 2004 the Mehdi Army took refuge in Iraq's holiest Shi'ite shrine, the Imam Ali Mosque in Najaf, during fighting with U.S. forces. A three-week siege was ended after a compromise under which the Shi'ite militiamen agreed to leave the shrine and U.S. forces pulled out of the city.

* Since the bombing of the Golden Mosque in Samarra unleashed a wave of sectarian bloodletting in February 2006, the Mehdi Army has grown more powerful. Sunni Arab leaders and U.S. officials blamed it for death squad killings.

* Sadr ordered the Mehdi Army to freeze its activities for six months in August 2007 after gun battles among rival Shi'ite factions killed dozens of people in the holy city of Kerbala. Sadr undertook the move to weed out rogue elements which have splintered away from the militia and to reassert his control. He extended the ceasefire by six months on February 22.

* The U.S. military praised Sadr for the truce and says it helped reduce violence in the second half of 2007, but U.S. forces have also pursued what they call "rogue" Mehdi Army elements, who they say are armed, trained and funded by Iran.

* Following deadly new clashes between Iraqi troops and the Mehdi Army, Prime Minister Nuri al-Maliki has threatened to bar Sadr's followers from provincial elections unless the militia is disbanded. A top aide to Sadr said the cleric planned to consult religious leaders on the matter and disband the militia if they order him to do so.

(Writing by David Cutler and Noah Barkin; Editing by Ibon Villelabeitia)