Iraqi forces battle for control of oil city

* Security forces fight militias in Basra

* Sadr threatens national "civil revolt"

* Fighting, curfews in southern towns

By Aref Mohammed

BASRA, Iraq (Reuters) - Iraqi security forces battled fighters loyal to Shi'ite cleric Moqtada al-Sadr in Basra on Tuesday in a drive to win control of the southern oil city, triggering a wave of violence in Baghdad and other cities.

Police and health workers said at least 12 people were killed in the fighting in Basra, which threatened a ceasefire Sadr imposed on the militia last August.

An Iraqi army commander said only that "many outlaws" had been killed in clashes in central and northern Basra where Sadr's Mehdi Army has a strong presence.

The operation, which Prime Minister Nuri al-Maliki said was aimed at "cleaning up" Basra of "criminals, terrorist forces and outlaws", provoked a fierce reaction from the Mehdi Army.

A hail of rockets and mortars hit U.S. military bases in Baghdad and the "Green Zone" government and diplomatic compound. One U.S. soldier was killed by a mortar attack on his patrol in northern Baghdad, a U.S. military spokesman said.