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

It said that British forces had returned fire and that a U.S. airstrike killed six militants behind the attack. The statement did not give the nationalities of the contractors.

Iraqi Prime Minister Nouri Maliki ordered a crackdown on militiamen in Basra at the end of March, mostly targeting fighters loyal to anti-U.S. Shiite cleric Muqtada Sadr.

After a bungled beginning to the campaign in which Sadr's Mahdi Army militia fought back and scores of civilians were killed, Iraqi forces have wrested control of most of the city. "This was the first indirect [rocket] fire attack causing casualties in Basra since March 27," the statement said.

Maliki, a Shiite Muslim, has turned his attention to pro-Sadr militants in Baghdad, particularly in the cleric's stronghold of Sadr City, where U.S. and Iraqi forces have been fighting daily street battles.