NAJAF, Iraq  — Shiite militiamen fired several mortar shells at a U.S. base in Najaf early Tuesday 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 U.S. Army said.

The sporadic overnight shelling 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 by U.S. retaliatory fire.

In Karbala, 50 miles north of Najaf, the city hall and the police headquarters, which are guarded by Bulgarian soldiers, came under mortar fire before dawn Tuesday, Bulgarian Defense Ministry spokeswoman Rumyana Strugareva said.

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.

At least 759 U.S. troops have died in Iraq since the war began in March 2003.

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.”

The Tuesday shelling was light compared to Monday’s sharp clashes.

White estimated that 20 militiamen were killed in the Monday battle — based on bodies and “watching young men fall after being hit.” He said there were few civilian casualties because troops were using precision fire.

New general takes over in Fallujah
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 and mutilation 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.

“The people of Fallujah should take pride in the fact that that mutilation was condemned from every (mosque) pulpit,” he said. “The people of Fallujah do not share responsibility for this prohibited act.”

Abdul-Latif met Tuesday with former officers of Saddam’s army at the headquarters of Iraqi security forces in Fallujah.

Fallujah residents have been celebrating what many see as a victory over the U.S. Marines, who are lifting a monthlong siege and pulling back from positions. The Marines are being replaced by an Iraqi force that is largely made up of former soldiers. U.S. officials have acknowledged they did not vet the force’s commanders for the extent of their ties to Saddam before letting the new brigade take over.

The new brigade has taken up positions in the south of Fallujah and is expected to replace Marines in the north and start patrols in the city soon.

U.S. officials say the Fallujah Brigade will crack down on hardcore guerrillas in the city even though the force itself will likely include some of gunmen involved in fighting against the Marines. Since Friday, masked and armed insurgents have moved freely in the city’s streets.

Clashes with Shiite militamen
In Najaf, U.S. troops fought for hours with Shiite militiamen Monday who barraged the U.S. base with mortars overnight, then in the afternoon opened fire from several directions. U.S. soldiers responded with heavy machine gun and tank fire.

The fighting subsided late Monday, but overnight, insurgents fired several more mortar rounds at the base.

The U.S. military moved soldiers to the base inside of the city after Spanish peacekeepers withdrew following a decision by the new Madrid government to terminate its involvement in Iraq.

U.S. troops took over the Spanish base as the coalition was cracking down on al-Sadr, who has taken refuge in Najaf, the holiest city in Shiite Islam.

Al-Sadr’s forces have stepped up attacks in recent days — apparently either to pressure U.S. officials to negotiate an end to the standoff or to goad troops into retaliating and raising Shiite anger.

The U.S. military has vowed to capture or kill the cleric, whose militia launched an uprising across the south in early April.

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