NAJAF, Iraq - U.S. forces and Shiite militants fought fierce battles in Najaf today, with airstrikes near a revered shrine and exchanges of mortar fire and artillery.

The U.S. military is stepping up pressure on the insurgents to quickly hand over the holy site to Shiite religious authorities.

Gunfire rang in the city throughout the day today, and black smoke rose over the Old City neighborhood, where much of the fighting has been centered and where militiamen loyal to radical cleric Muqtada al-Sadr still control the Imam Ali Shrine. Militiamen early today fired mortars at U.S. troops, who responded with artillery.

In Baghdad's heavily Shiite Sadr City neighborhood, an explosion, apparently from a U.S air attack, killed four people and injured nine others today, said Dr. Qasim Saddam, director of Sadr Hospital. The U.S. military said it was unaware of the incident.

In the southern city of Nasiriyah, U.S. journalist Micah Garen said after his release from more a week in captivity that he hoped to stay in Iraq to continue working on a documentary project he'd started about the looting of archaeological sites.

"This experience hasn't made me want to leave at all," Garen said late Sunday in an interview with Associated Press Television News. He also thanked al-Sadr for helping free him from his captors.

Late Sunday, U.S. warplanes and helicopters attacked positions in Najaf's Old City for the second night, witnesses said. Leaders of al-Sadr's Mahdi Army militia said the Imam Ali Shrine compound's outer walls were damaged in the attacks. The U.S. military said it had fired on sites south of the shrine, from which militants were shooting, and did not hit the compound wall.

The renewed clashes in Najaf appeared more intense than in recent days as U.S. forces sealed off the Old City. But Iraqi government officials counseled patience, saying they intended to resolve the crisis without raiding the shrine, one of Shia Islam's holiest.

Worries over the fallout have fueled calls for international action to end the Najaf fighting. Syria's prime minister, Naji al-Otari, in talks with his Jordanian counterpart today, warned that instability in Iraq "is about to backfire on neighboring countries" and called for Arabs and Iraq's neighbors to "help it get out of its current ordeal."

Iran has called on Muslim nations to hold an urgent meeting to deal with Iraq. Iranian President Mohammad Khatami repeated denials of claims by some Iraqi officials that his country supports al-Sadr.

The chairman of the world's largest grouping of Muslim countries suggested the United Nations take a role in helping stop the Najaf violence. "If the confrontation in Najaf is not defused, it will inflame emotions and may create unpredictable conditions," said Malaysian Prime Minister Abdullah Ahmad Badawi, who heads the 57-member Organization of the Islamic Conference.

Assailants in Tikrit, 80 miles north of Baghdad, killed one Turkish citizen and two Iraqis along a road as they headed to the northern city of Kirkuk late Sunday, Maj. Neal O'Brien, a spokesman for the U.S. Army's 1st Infantry Division, said Monday. No other details of the attack were immediately available.

In Kirkuk, Sharzad Hassan, 31, an official with the pro-U.S. Patriotic Union of Kurdistan was gunned down by unknown attackers late Sunday in a drive-by shooting, police officer Sarhat Qadir said today.

Five U.S. troops were reported dead on Sunday - including three Marines killed in action Saturday in Anbar province, a Marine who died in a vehicle accident and a soldier killed by a roadside bomb Sunday in the northern city of Mosul.

As of Friday, 949 U.S. service members have died since the beginning of military operations in Iraq in March 2003, according to the U.S. Defense Department.

Garen, who was kidnapped Aug. 13 in Nasiriyah, was released Sunday along with his Iraqi translator at al-Sadr's offices there after the cleric's aides appealed for his freedom.

Sheik Aws al-Khafaji, an al-Sadr aide, said Sunday night the kidnappers mistakenly had thought Garen was working for the U.S. intelligence services.