So much for thesuccess of the surge.

Fighters from the Shi’ite Mehdi Army and U.S. soldiers exchanged rocket and mortar fire on Thursday, threatening a ceasefire declared by the militia’s leader, the anti-U.S. cleric Moqtada al Sadr.

Sadr, whose militia fought two battles against U.S. forces in southern Iraq in 2004, extended a ceasefire last month, but at the weekend issued a statement telling followers they could defend themselves if attacked.

The ceasefire has been praised by U.S. commanders for reducing violence, with attacks across Iraq down by 60 percent since last June. But U.S. forces are stretched thin by an increase in attacks in Baghdad and northern Iraq since January.

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An Iraqi police official, who asked not to be identified, said as many as 11 Katyusha rockets landed on the U.S. base near Kut, 170 km (105 miles) southeast of Baghdad late on Wednesday.