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

North of Baghdad, four U.S. soldiers from the 1st Infantry Division 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, U.S. commanders said they would maintain the current level of 135,000 American troops in Iraq through the end of next year because of increased anti-coalition violence. Generals had planned to cut the number of occupying troops to about 115,000.

The announcement 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 the 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.

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.

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.

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.

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.

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.