As the massive search for three U.S. soldiers feared captured by Al Qaeda entered its sixth day Thursday, the military said the ambush on their convoy apparently confiscated the dog tags of one of the four American soldiers who died in the attack.

An explosion also rocked the Green Zone on Thursday in the third attack in three days on the heavily fortified area in central Baghdad where the U.S. Embassy is located and Iraq's government and parliament meets.

Maj. Webster Wright, a spokesman for the Second Brigade of the 10th Mountain Division the unit that was attacked and that is leading the search for the three missing soldiers, said Saturday's assault in an insurgent stronghold south of Baghdad apparently used rocket-propelled grenades and small arms fire to severely damage the two Humvees in the stationary U.S. convoy.

He said the dog tags of one of the four American soldiers who died were missing and apparently had been taken from the scene by the attackers. That could explain why the military has only been able to identify three of the four dead U.S. soldiers.

About 4,000 U.S. troops and 2,000 Iraqis are searching for the three U.S. soldiers feared captured by Al Qaeda during the ambush, which also killed one Iraqi soldier.

Maj. Gen. Rick Lynch, commander of U.S. troops south of Baghdad, said Wednesday that the U.S. was offering rewards of up to US$200,000 for information on the missing soldiers' whereabouts.

"We've done so much as to drain canals after a report that the bodies were in a canal," Lynch told The Associated Press. "So we're leaving no stone unturned."

Lynch said he was optimistic that the three soldiers would be found alive, and the search remained focused on the area where they went missing in a rural area near the town of Youssifiyah, 12 miles south of Baghdad.

Wright said Cargouli, a Sunni tribe, dominates the area, where many former intelligence agents, Baathists and Republican Guards from Saddam Hussein's government live.

An Al Qaeda front group, the Islamic State of Iraq, has said it captured the U.S. soldiers and warned the Americans in a Web statement on Monday to call off the hunt "if you want their safety."

Meanwhile, attacks by suspected insurgents continued in other parts of Iraq.

U.S. Embassy spokesman Lou Fintor said at least one explosion occurred about 2:15 p.m. in the Green Zone and U.S. forces were trying to determine whether it was a mortar or a rocket attack. He had no immediate information on casualties.

That came a day after about a dozen mortar rounds hammered the sprawling complex , killing at least two Iraqis, wounding about 10 other people and raising new fears for the safety of workers at the nerve center of the American mission in Iraq.

Terrified pedestrians raced for the safety of concrete bunkers. Motorists abandoned their cars and sprinted for cover. Sirens wailed and loudspeakers warned people to seek safety.

Nine people also were wounded in a rocket strike on the 3.5-square-mile (9-square-kilometer) area on Tuesday, but no American casualties were reported in either attack.

For the second time in a week, a bomb also struck a bridge in southeastern Baghdad on Thursday, killing two civilians and wounding five, police said.

Last Friday, a large fuel truck barreled toward a checkpoint at the new Diyala Bridge and blew up, killing about a dozen people, police said.

The bridge, which crosses the Diyala River, a Tigris tributary, also was damaged, and the attack set fire to police and civilian cars driving across it.

Since then, the bridge has been closed to traffic, and Iraqis have been walking across it toward central Baghdad, many to board buses and continue their travels.

Attackers also blew up a bridge that connects an area west of the northern city of Mosul with the autonomous Kurdish region late Wednesday, causing it to collapse but causing no injuries, police said.

At 7:30 a.m. on Thursday, a roadside bomb exploded near the entrance to the bridge, killing at least two Iraqi pedestrians and wounding five, police said.

At about the same time, a bomb hidden near the home of an Iraqi police officer in Hillah city, 60 miles south of Baghdad, exploded as he left for work, killing him, police said. Shrapnel also smashed a window in the house, wounding three of his children, police said.

Insurgents and militias often target Iraqi policemen and soldiers, accusing them of being collaborators with the U.S.-led coalition.

In northern Baghdad, about 200 Iraqis marched down a street in the mostly Shiite neighborhood of Shaab, shouting slogans and carrying banners demanding that the thousands of U.S. soldiers conducting a security crackdown in the capital stop creating forward operating bases in neighborhoods and searching homes for suspected insurgents and militiamen.

In other violence on Thursday:

—Fighting broke out in Diwaniyah, 80 miles south of Baghdad, between police and the Madhi Army militia, which is led by the radical cleric Muqtada al-Sadr, killing four civilians and wounding four, police said.

—A poor Iraqi man searching through trash for tin cans in the city of Najaf, 100 miles south of Baghdad, died when a mortar round left there exploded, police said.

—A mortar round hit a factory behind a mosque near the Shiite slum of Sadr City in Baghdad, killing one worker and wounding four, police said.

—The bodies of seven handcuffed and tortured Iraqi men were found, four in Baghdad and three south of the capital, police said. They apparently were the latest victims of death squads that operate in Iraq, motivated by sectarian hatred or the search for ransoms.