NAJAF, Iraq - Shiite militiamen fired several mortar shells at a U.S. base in Najaf early today and at a city hall guarded by Bulgarian troops in another Shiite city. Elsewhere, four U.S. soldiers died after their Humvee overturned during a combat patrol.

The sporadic overnight shelling of the U.S. base in Najaf followed intense attacks Monday by militiamen loyal to radical cleric Muqtada al-Sadr, who pounded the base with mortars and fired rifles and machine guns. No U.S. soldiers were killed in either attack. U.S. officers estimate about 20 Iraqis were killed.

In Karbala the city hall and the police headquarters, which are guarded by Bulgarian soldiers, came under fire before dawn today.

No casualties or damage were reported after that attack which lasted about ten minutes. Al-Sadr's forces have battled coalition troops since the occupation authorities sought his arrest last month for the killing of a rival cleric last year.

North of Baghdad, four U.S. soldiers of the 1st Infantry Division died from injuries sustained when their vehicle rolled over Monday night during a patrol near Khalis, some 40 miles northeast of Baghdad, the military said in a statement.

In Najaf, U.S. commanders said they are holding back fire to avoid serious clashes in the city, which is home to one of the most holy Shiite Muslim shrines.

"I think every soldier here understands the sensitivities of the situation," said Lt. Col. Pat White. He added that U.S. troops would "maintain our defense posture" until someone "much, much higher than me makes a different decision."

In Fallujah, Maj. Gen. Mohammed Abdul-Latif, who opposed Saddam Hussein, took over as head of a new force that will replace U.S. Marines in the city, subject to a final background check by U.S. officials. Abdul-Latif would replace another general who may have been involved in Saddam-era repression.

During a brief news conference in Fallujah, Abdul-Latif condemned the

brutal

killing of four American contractors there last month, which triggered the three-week siege of the city. However, Abdul-Latif said the people of Fallujah collectively were not to blame.