FACTBOX: What is the Mehdi Army?

(Reuters) - Iraqi security forces fought fierce gunbattles with powerful Shi'ite militias in Basra on Tuesday in a major operation aimed at bringing the southern oil city under government control.

Security forces targeted six districts in central and northern Basra where the Mehdi Army militia of Shi'ite cleric Moqtada al-Sadr has a strong presence.

Here are some details about the Mehdi Army:

* 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 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 gunbattles 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.

(Writing by David Cutler, London Editorial Reference Unit)