Elsewhere, four U.S. soldiers died after their Humvee overturned during a combat patrol, the U.S. Army said.
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 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.
In major developments:
Since photographs of abused prisoners surfaced in January, five investigations have been launched, the Pentagon says. Seven soldiers have received civil sanctions, and six others face criminal charges. Separate inquiries are under way concerning interrogations, guard training and overall detention policy in Iraq and Afghanistan.
The U.N. special investigator on torture, Theo van Boven, issued a statement late Monday expressing serious concern about the reports of torture and "other cruel, inhuman or degrading treatment" of Iraqis detained by U.S. and British forces.
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, when militiamen 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.
White estimated that 20 militiamen were killed in the Monday battle.
The U.S. military, cracking down on al-Sadr, moved soldiers to the base inside of the city after Spanish peacekeepers withdrew.
The U.S. military has vowed to capture or kill the cleric, whose militia launched an uprising across the south in early April. Al-Sadr's forces have stepped up attacks in recent days, and a spokesman for al-Sadr warned the United States not to erect checkpoints in Najaf, AFP reported.
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.
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.
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. Some commanders previously said Maj. Gen. Jassim Mohammed Saleh would take charge of the Fallujah brigade, but his apparent Republican Guard service eventually disqualified him.
During a brief news conference in Fallujah, the new commander of the U.S.-backed Iraqi force, 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."