U.S. troops engaged in sporadic fighting with the Mahdi Army, a militia loyal to the Shiite cleric Moqtada Sadr, throughout much of the day on Saturday.

Around midnight, witnesses in Najaf, about 90 miles south of Baghdad, said Sadr militants attacked a U.S. military base outside town with mortar fire and sought refuge near the Imam Ali shrine, among the holiest sites in Shiite Islam.

Witnesses described mortars falling randomly among the houses and shuttered shops. The police station and the provincial governor's offices also came under attack. Just after midnight, U.S. tanks from the 1st Armored Division entered Kufa, a town just east of Najaf where Sadr delivers his regular Friday sermon. Witnesses said U.S. attack helicopters provided air support for the push into the city.

The fighting flared despite fresh attempts by influential tribal leaders to end the weeks-old military standoff between U.S. forces and Sadr, whom U.S. officials want to arrest on murder charges. Sadr, 31, is demanding that U.S. forces leave the holy cities and has called on all Iraqis to rise up against the U.S. occupation. His aides said they met Saturday with several tribal sheiks.

"These are new negotiations to end the crisis everywhere and by both sides," Qays Khazali, a Sadr spokesman, said from Najaf. "There should be a choice to end this crisis."

Karbala, a Shiite holy city farther north, remained calm after days of combat between Mahdi Army fighters and U.S. soldiers.

Special correspondent Saad Sarhan in Najaf contributed to this report.