The soldiers took control of the building in Najaf without a fight, but heavy gunfire was heard after they moved in and smoke rose over the city. Motorists fled the area through deserted streets, honking their horns. Fighters loyal to cleric Muqtada al-Sadr had controlled the building since they launched their rebellion April 4.
Fighting east of Najaf killed an estimated 40 militiamen around Kufa, said Capt. Roger Maynulet, commander of a tank company with the Army's 2nd Armored Cavalry Regiment. U.S. officers said American forces were sent to the east of the city to draw militia fighters away from the governor's office.
In Karbala, U.S. troops backed by tanks attacked al-Sadr's militiamen, the Arabic language television station Al-Jazeera reported. The station, quoting its correspondent in the city, said explosions were heard in various parts of Karbala.
Chief U.S. administrator L. Paul Bremer announced the appointment of a new governor for Najaf province as part of the campaign to crack down on al-Sadr's militiamen.
In Baghdad, a suicide attacker detonated a car bomb outside the so-called Green Zone that houses the U.S. headquarters, killing five Iraqi civilians and a U.S. soldier. Also Thursday, the U.S. command said that two U.S. soldiers were killed and two were wounded when a roadside bomb exploded in Baghdad.
In other developments:
The Baghdad car bomb injured 25 people, including two American soldiers. The bomb, hidden inside an orange-and-white taxi, exploded outside a 3-foot-high concrete blast wall protecting a U.S. checkpoint.
The U.S. soldier who died in the car bombing was the 21st U.S. serviceman killed in Iraq in May. The injured included two U.S. soldiers and three Iraqi policemen, the U.S. military said.
The suicide bomber also died in the attack, the military said.
A statement on a Web site known for militant Islamic messages and signed by a group linked to al Qaeda claimed responsibility. The message was signed by the "military wing" of the "Monotheism and Jihad Group," which is believed led by Abu-Musab al-Zarqawi.
Al-Zarqawi, a Jordanian, is wanted by the United States for allegedly organizing terrorists to fight U.S. troops in Iraq on behalf of al Qaeda. U.S. officials have offered a $10 million reward for al-Zarqawi's capture.
Hours after the car bomb, a roadside bomb exploded on Saadoun Street, a busy commercial avenue on the east side of the Tigris River near the Palestine and Sheraton hotels. Two Iraqis were injured.
The attacks came as U.S.-led forces increased pressure on al-Sadr. In the past two days, an estimated 76 militiamen loyal to him have been killed in gunbattles and raids, including Thursday's fighting.
"I think we are going to gain momentum from now on," said Lt. Michael Watson, a platoon leader with the 2nd Armored Cavalry Regiment in Najaf.
In Najaf, soldiers moved into the area of the governor's office from four directions and took control of the building without a fight. Afterward, gunfire could be heard and U.S. helicopters were seen flying low over parts of the city.
Al-Sadr's forces last month seized government buildings and police stations in southern cities, including Najaf and Kufa.
In Baghdad's Sadr city, an overwhelmingly Shiite Muslim enclave and al-Sadr stronghold, militiamen ambushed four U.S. patrols. Ten attackers were killed in retaliatory gunfire, a U.S. officer said, speaking on condition of anonymity. There were no reports of U.S. casualties.
Moderate Shiites tried to persuade al-Sadr to back away from his confrontation with the United States — a reflection of their growing concern.
In Baghdad, Bremer announced the appointment of Adnan al-Zurufi as governor of Najaf province. Bremer said the "the difficulty and suffering of the people" of the region "cannot continue."
The region's previous governor, Haider Mayali, went to Iran on a visit and never returned.
A senior U.S. diplomatic official said the occupation leaders are eager to dislodge al-Sadr's militia — and the Sunni Muslim rebels that still occupy the western city of Fallujah — before the June 30 handover of some sovereignty to an Iraqi government, after which the U.S. mandate to mount such actions is unclear.