Najaf, Iraq ? U.S. soldiers seized the governor’s office Thursday in the holy city of Najaf, wresting control from Shiite militiamen loyal to a radical cleric in battles that left an estimated 40 insurgents dead.

In Baghdad, a suicide car bomber killed five Iraqi civilians and an American soldier at the edge of the heavily guarded Green Zone that houses the U.S. headquarters. The blast wounded 25 people, including two U.S. soldiers.

Amid concerns that U.S. troops were about to move directly against anti-American cleric Muqtada al-Sadr, his militiamen dug in at Najaf, taking positions behind earthen mounds leading into the city center and firing a barrage late Thursday at the U.S. base here.

American soldiers responded with heavy 120mm mortar fire. U.S. jets streaked across the blackened night sky, and two large plumes of smoke could be seen rising over the east bank of the Euphrates.

Earlier in Najaf, gunmen took cover behind buildings as American helicopters flew overhead. An increased number of fighters were seen in the city center as the militia attempted to regroup after losing the governor’s office.

“We will fight until the last drop of our blood,” said Dhia Shami, as he stood behind a dirt barricade.

“We expect the Americans to retreat,” said fellow militiaman Malek Holeicha. “We are fighting for our faith. They don’t have any faith.”

Iraqis look through a shattered window in downtown Baghdad after an explosion on a busy commercial avenue. In a separate incident Thursday, a suicide car bomb exploded near the main complex housing U.S. administrative offices in central Baghdad, wounding 25.

At the city’s Imam Ali shrine, one of the holiest sites in Shiite Islam, a coffin wrapped in an Iraqi flag was brought in — apparently one of the dead from the fighting.

“This is a martyr for Muqtada,” mourners chanted.

The Najaf fighting came as chief U.S. administrator L. Paul Bremer announced the appointment of Adnan al-Zurufi as governor of Najaf province, a step that is part of the campaign to regain control of the area from al-Sadr’s militiamen. On April 4 his al-Mahdi Army seized government buildings and police stations throughout the province.

The United States is planning to hand some sovereignty to a transitional Iraqi government June 30 and is looking to end al-Sadr’s uprising, which has left parts of southern Iraqi outside of coalition control.

U.S. troops have been stepping up assaults on al-Sadr’s followers, raiding buildings where weapons are stored in several southern cities in increasingly bloody skirmishes. Moderate Shiite leaders have called on al-Sadr’s militiamen to abandon Najaf, isolating the cleric and making it easier for U.S. forces to act against him.