FACTBOX - Main players in Iraq's Basra

(Reuters) - Iraqi security forces battled the Mehdi Army militia in Basra on Tuesday in a drive to win control of the southern oil city, but violence appeared to be spreading to Baghdad and other cities.

Two powerful factions of Iraq's Shi'ite majority, the Supreme Islamic Iraqi Council and the Mehdi Army militia of Moqtada al-Sadr, are fighting for power in Basra along with a smaller Shi'ite party, Fadhila.

Here are some details on the main players:

* SADR MOVEMENT:

-- Loyalists of Shi'ite cleric Moqtada al-Sadr are widely seen as the most influential group on the streets of Basra. Sadr's political movement and Mehdi Army militia have popular support. Critics accuse them of using violence to impose strict Islamic rules, a charge Sadrists deny.

-- The Sadrists recently signed a truce with other major Shi'ite parties, agreeing that militia members would not carry guns openly as long as security forces do not target them.

-- Unlike the Supreme Islamic Iraqi Council, Sadr opposes the idea of federalism for the south.

-- The militia has kept a low profile since Sadr called a ceasefire last August and extended it last month.

-- But gunbattles in Baghdad and the southern city of Kut last week have raised fears that it may be unravelling at a time when the U.S. military is withdrawing 20,000 troops.