The death brought to 49 the number of soldiers killed in a guerrilla war since May 1, when President Bush declared an end to major combat in Iraq. In all, 163 U.S. soldiers have died in action in Iraq, 16 more than in the 1991 Gulf War.
It took the military hours to confirm the attack. Witnesses had given reporters varying accounts earlier in the day, saying at least three U.S. soldiers were injured or killed.
"I saw at least two injured soldiers, then I saw the third one who was thrown out of the car. They (others soldiers) pulled him under the car," said witness Alim Naati.
Shihab Ahmed, who owns a nearby flower shop, said he was told all the soldiers died. The witnesses said the victims were in canvas-top Humvee moving along Palestine Street in central Baghdad.
In a separate incident, a U.S. soldier was killed in a vehicle accident near the southern city of Nasiriyah. Another soldier was injured in the accident, reported by U.S. Central Command. The military provided no other details.
North of Baghdad, on the road from Baqouba to Tikrit, insurgents floated a bomb down the river on a palm log and detonated it under a bridge the military had been repairing.
It was believed to have been the first such attack by insurgents on a bridge. The structures are vital to a country with two major rivers, the Tigris and the Euphrates. The military closed a pontoon crossing down river that had been open for civilian traffic during the repairs.
"Because of this damage we've got to shut it to all the civilian traffic effective today," Lt. Col. Bill Adamson, a 4th Infantry Division commander, said.
The bridge was a major link over the Diala River, a Tigris tributary, carrying traffic between the restive cities of Baquoba and Tikrit, both hotbeds of resistance in the so-called "Sunni Triangle." The region, stretching north and west from Baghdad is a major center of support for Saddam.
In other developments:
At least twice in the past week, American soldiers have raided houses where they believed they may have missed Saddam by less than 24 hours — once in the northern city of Mosul and once at a farmhouse near Tikrit, Saddam's hometown and power base.
The U.S. military would not confirm a raid in Baghdad's upscale Mansour neighborhood Sunday evening. Witnesses said soldiers shot their way into the home of Prince Rabiah Muhammed al-Habib, one of Iraq's most influential tribal leaders.
The prince, who wasn't at home at the time of the raid, told The Associated Press that he believed the Americans were looking for Saddam.
"I found the house was searched in a very rough way. It seems the Americans came thinking Saddam Hussein was inside my house," al-Habib said. He didn't elaborate.
U.S. soldiers shot at several cars and bystanders that approached the mansion during the raid, witnesses said, and one hospital reported at least five Iraqis were killed.
That raid came hours after troops of the 4th Infantry Division moved in on three farms in the Tikrit area in search of Saddam's new security chief, and perhaps the ousted dictator himself.
"We missed him by 24 hours," said Lt. Col. Steve Russell, who led the operation that was witnessed by an Associated Press reporter.
Hundreds of soldiers, backed by Bradley fighting vehicles, surrounded the farms as Apache attack helicopters hovered above. No shots were fired as about 25 men emerged from the houses peacefully. They were detained briefly and released.
The raid was prompted by Thursday's capture in Tikrit of a group of men believed to include as many as 10 Saddam bodyguards. Soldiers learned from them that Saddam's new security chief — and possibly the dictator himself — were staying at one of the farms, Russell said.
"The noose is tightening around these guys," said Col. James C. Hickey, a brigade commander. "They're running out of places to hide, and it's becoming difficult for them to move because we're everywhere. Any day now we're going to knock on their door, or kick in their door, and they know it."
Gen. Richard Myers, chairman of the Joint Chiefs of Staff, visited 4th Infantry commanders in Tikrit on Sunday and later told reporters in Baghdad that Saddam was too busy "trying to save his own skin" to lead an insurgency against American forces.
"He is so busy surviving he is having no impact on the security situation here," Myers said. "It's a big country, but we'll find him."
Myers met with Lt. Gen. Ricardo Sanchez, commander of U.S. ground forces in Iraq and planned to leave Monday.
With U.S. soldiers standing guard, workers used jackhammers to begin demolition of the villa in northern Mosul where Odai and Qusai Hussein were killed. Chunks of masonry fell to the ground, and the area filled with debris.
Residents passed by asking for souvenirs, but the soldiers told them that was out of the question.
After the firefight on Tuesday, intelligence sources reported Saddam was at a different location in the same city. Elements of the 101st Airborne Division mounted another raid, a military official familiar with the operation told AP.
"We missed him by a matter of hours," the official said on condition of anonymity.
Another U.S. soldier was killed south of Baghdad on Sunday.