BAGHDAD, May 8 (Reuters) - Militants fired rockets into a British forces base in Iraq's southern oil town of Basra on Thursday, killing two contractors and wounding four other civilians, the U.S. military said on Thursday.

It said British forces had returned fire and U.S. forces killed six militants behind the attack in an air strike. The statement did not give the nationalities of the contractors.

Iraqi Prime Minister Nuri al-Maliki ordered a crackdown on militiamen in Basra at the end of March, mostly targeting fighters loyal to anti-American Shi'ite cleric Moqtada al-Sadr.

After a bungled beginning to the campaign in which Sadr's Mehdi Army fought back and scores of civilians were killed, Iraqi forces have successfully wrested control of most the city. Basra's formerly lawless streets are now largely quiet.

"This was the first indirect fire (rocket) attack causing casualties in Basra since March 27," the statement said.

Maliki, a Shi'ite, has turned his attention to Sadrist militants in Baghdad, particularly in the cleric's stronghold of Sadr City, where U.S. and Iraqi forces have been fighting daily street battles in the crowded slum of 2 million. (Reporting by Tim Cocks; Editing by Janet Lawrence)

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