Shiite militiamen fired mortars at a U.S. base in Najaf (search) and bombarded a municipal hall in a nearby city Tuesday, as U.S.-led forces sought to resolve their standoff with militants south of the capital.

North of Baghdad (search), four U.S. soldiers from the 1st Infantry Division (search) were killed after their Humvee overturned during a combat patrol north of Baghdad, the Army said.

Sporadic overnight mortar attacks on the U.S. base in Najaf followed intense fighting Monday between American forces and militiamen loyal to radical cleric Muqtada al-Sadr. As many as 20 Iraqis were killed Monday. No coalition troops died.

In Washington, defense officials said Tuesday an expanded force of troops will stay in Iraq beyond June because of increased anti-occupation violence.

U.S. military commanders will send 10,000 Army and Marine Corps troops for one-year tours. Also, the Army plans to announce that about 37,000 National Guard and Reserve troops will be called to support three National Guard combat brigades being sent to Iraq this winter, the officials said, speaking on condition of anonymity.

There are about 138,000 U.S. troops in Iraq. That number was to have fallen to about 115,000 this spring; the latest change of plans would leave the total at about 125,000 to 128,000 after June.

Moves maintain U.S. forces in Iraq came amid a scandal over alleged abuse of Iraqi inmates in coalition jails, centered on the notorious Abu Ghraib prison near Baghdad. The apparent humiliation of prisoners photographed in Abu Ghraib has drawn worldwide condemnation and fueled anti-American feelings in the Arab world.

Seeking to limit the damage, U.S. National Security Adviser Condoleezza Rice took to Arab airwaves Tuesday to appeal for trust from a skeptical public.

"We have a democratic system that holds people accountable for their actions," Rice said on the popular Al-Jazeera satellite television station, widely seen across the Arab world and by Arab and Muslim communities elsewhere.

President Bush "guarantees that those who did that be held accountable," she said in remarks dubbed into Arabic by the station.

In Najaf, where the U.S. military moved in last month after Spanish peacekeepers withdrew from Iraq, al-Sadr's forces have stepped up attacks in recent days. Their assaults seem aimed either at pressuring U.S. officials to negotiate an end to the standoff or goading troops into a heavy retaliation that would inflame Shiites.

The military has been wary of sparking broader fighting. Al-Sadr, who launched his uprising in April, has an office near Iraq's holiest Shiite shrine.

Militants in Karbala shelled the city hall and police headquarters before daybreak. Both are guarded by Bulgarian soldiers; no casualties were reported.

Near the northern city of Mosul, insurgents blasted a convoy of American soldiers with a homemade bomb, sending up shrapnel that slightly wounded three soldiers. Troops shot and killed two men who set off the bomb, the military said.

Meanwhile, Iraqis were patrolling Fallujah, taking over for Marines pulling back from the city where U.S. forces have been battling Sunni insurgents.

A senior Marine officer said the new Iraqi force, which was swiftly formed with U.S. backing and will eventually number up to 1,100 troops, is "meeting expectations" in bringing calm to the city, where a nearly monthlong siege left 10 Marines and several hundred Iraqis dead.

Col. John Coleman, chief of staff for the 1st Marine Expeditionary Force, said "there is a percentage of (the city) where normalcy has returned."

Maj. Gen. Mohammed Abdul-Latif, who opposed former dictator Saddam Hussein, was preparing to take over as head of the new force in Fallujah, west of Baghdad, subject to a final background check.

The U.S. move to have Abdul-Latif lead the Fallujah Brigade came amid complaints from some Iraqis that Maj. Gen. Jassim Mohammed Saleh, a former member of Saddam's Republican Guard, may have been involved in past repression by the ousted regime.

Hoshyar Zibari, Iraq's Kurdish foreign minister, said there were reports Saleh was involved in crushing the 1991 uprising by Kurds.

"The vetting was imperfect," Defense Secretary Donald Rumsfeld said at the Pentagon. "There was enough question that the people again on the ground that a different individual, Gen. Latif, would be preferable and less risky."

At a news conference, Abdul-Latif condemned the killing and mutilation of four American contractors in Fallujah on March 31, but said residents shouldn't bear collective blame.

Marines laid siege to the city shortly after the burned bodies were dragged through the street and two corpses strung up from a bridge over the Euphrates River.

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

U.S. officials say the Fallujah Brigade will crack down on die-hard guerrillas even though the force itself will likely include some gunmen who had been involved in battling the Marines.