KARBALA, Iraq - American AC-130 gunships and tanks pounded militia positions early today 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. Hospital officials said the dead included two Iranian pilgrims.

Meanwhile, an Iraqi security official said today that Iraqi police have arrested four people in the killing of American Nicholas Berg.

The suspects were former members of Saddam Hussein's Fedayeen paramilitary organization, said the official, who spoke on condition of anonymity. They were arrested a week ago in a house in Salaheddin province, north of Baghdad.

In Karbala, U.S. forces withdrew from a mosque in the city center that had been used by insurgents as a base of operations, but said patrols in the city would continue.

Fighting between American forces and cleric Muqtada al-Sadr's militia was also heavy in Najaf and neighboring Kufa, south of Baghdad. Explosions rocked the center of Najaf, near local government buildings, and today prayers were canceled because of the violence.

Also today, 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 and dropped them off in Baqouba, north of the capital.

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

The group that was involved in the killing of Berg was led by Yasser al-Sabawi, a nephew of Saddam Hussein, the security official said. He said American intelligence had asked Iraqi authorities to hand over the suspects, but they were still in Iraqi hands.

Al-Sabawi was not among those arrested, the Iraqi official said.

American officials have said they believe Abu Musab al-Zarqawi, a Jordanian wanted for allegedly organizing terrorists to fight U.S. troops in Iraq on behalf of al-Qaida, carried out Berg's killing.

The body of Berg, 26, was found May 8 near a highway overpass in Baghdad. He was last seen on April 10 when he left his Baghdad hotel.

Meanwhile, a Spanish National Radio reporter was taken captive by insurgents today while traveling to report on al-Sadr's sermon to about 15,000 worshippers at prayers in the city of Kufa, the station's news director said in Madrid.

Correspondent Fran Sevilla was seized by militia fighters loyal to al-Sadr, news director Javier Arenas said. Sevilla was being held in a mosque and apparently had not been harmed, Arenas said.

Near Baqouba, north of Baghdad, gunmen in pickup trucks opened fire today 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.

In Madrid, the Defense Ministry said the last Spanish troops in Iraq withdrew from their base in Diwaniya today and headed for Kuwait on their way back home. The news agency Efe said the Spanish general in charge of the unit transferred control of so-called Base Espana in south-central Iraq to U.S. forces Thursday night.