KARBALA, Iraq - American AC-130 gunships and tanks pounded militia positions early Friday near two shrines in the center of the holy city of Karbala, and the U.S. military said it killed 18 fighters loyal to a rebel cleric.

Also Friday, the U.S. military released a group of prisoners from the Abu Ghraib jail, center of a scandal involving abuse of detainees by American soldiers. Several buses carrying prisoners left the prison on the western outskirts of Baghdad. It was unclear where they were headed.

Earlier this week, Brig. Gen. Mark Kimmitt said the military planned to release 472 prisoners on Friday.

The military periodically frees prisoners from Abu Ghraib, which was also notorious as the site of executions and torture during Saddam Hussein's regime.

The fighting started after insurgents fired several rocket-propelled grenades at U.S. tanks that were patrolling on the outskirts of Karbala's so-called "Old City," a maze of alleyways and cluttered buildings, said U.S. Army Col. Pete Mansoor of the 1st Armored Division.

The tanks returned fire, and more than two hours of heavy fighting followed. Smoke billowed from burning buildings. Explosions lit up the night sky and reverberated throughout the city. Electric lights flickered on and off. By 3 a.m., the fighting had stopped.

A rebel weapons cache was hit, the military said.

Much of the fighting was near the city's Imam Hussein and Imam Abbas shrines, which U.S. forces allege are being used by militiamen as firing positions or protective cover. Mansoor said the shrines were not damaged.

The military says it is doing its best to avoid damage to the gold-domed shrines, which could infuriate Shiite Muslims who are not involved in the conflict. Cleric Muqtada al-Sadr, who launched an uprising against the U.S.-led occupation of Iraq last month, has accused U.S. forces of desecrating holy sites.

Mansoor said 18 insurgents died.

Dr. Abbas Falih al-Hassani of Karbala's al-Hussein hospital said 12 people died, including two Iranian pilgrims. Thirteen were injured.

The dead included a driver for a camera crew of the Al-Jazeera television network, the station reported.

Rashid Hamid Wali, 40, died while assisting a crew from the Qatar-based network that was filming the clashes from a hotel roof shortly after midnight, said Ahmed al-Sheikh, the network's news editor in Qatar.

It was unclear who was responsible.

Early Friday, U.S. troops pulled out of the Mukhayam mosque in Karbala, the scene of fierce fighting last week during which coalition forces ousted insurgents who were using it as a base of operations. Since then, American soldiers there have come under frequent attack.

The military said it planned to conduct regular patrols in Karbala despite the withdrawal from the mosque.

Officers said soldiers operating in Karbala's Old City in the past two weeks experienced intense urban combat similar to battles in Mogadishu, Somalia, more than a decade ago.

The mosque pullout happened hours after a major military operation into Karbala was postponed. The operation was postponed to allow discussion between Iraqi leaders and al-Sadr's militia on a possible negotiated end to the fighting, a senior military official said on condition of anonymity.

The U.S. military says al-Sadr, who is wanted in the murder of a rival moderate cleric last year, must disband his militia. Al-Sadr has urged his fighters to resist U.S. forces and he demands an end to the occupation.

There was also overnight fighting between U.S. forces and al-Sadr loyalists in another holy city, Najaf. One civilian died and another was injured when their car was caught in the crossfire, hospital officials said.

In the northern city of Kirkuk, American troops detained a representative of al-Sadr, Sheik Anwar al-Jinani and 10 supporters at a mosque, Iraqi authorities said. The press office of the U.S.-led coalition in Baghdad said it had no information.

Near Baqouba, north of Baghdad, gunmen in pickup trucks opened fire Friday on a base of the Iraqi security forces, killing four, Iraqi authorities said. The slain men were members of the U.S.-backed Iraqi Civil Defense Corps.

Insurgents often target Iraqis who are perceived as collaborators with the coalition.