BAGHDAD, Iraq, Aug. 3

Fourteen marines were killed Wednesday when their troop carrier was blown up by a huge roadside bomb in the western town of Haditha, in one of the deadliest attacks on American troops since the invasion in March 2003. An Iraqi civilian interpreter working with the marines was also killed in the blast.

American officers said the marines had been riding in an amphibious troop carrier during combat operations on the southern edge of the city at dawn on Wednesday when the bomb exploded. The blast flipped the 25-ton troop carrier and caused it to burst into flames. One marine was seriously wounded; everyone else on board died.

The troop carrier, which looks like a large boat with wheels, was designed for amphibious assaults and carries only light armor. Even so, the vehicles are the marines' primary means of transporting troops and cargo across the desert landscape of western Iraq.

The attack brought the number of marines killed in Haditha to 20 in less than two days. On Monday, guerrillas ambushed and killed a group of six American snipers who were moving through the area on foot. The snipers had been deployed to hunt down and shoot insurgents trying to plant roadside bombs like the one that exploded Wednesday. Most of the dead have been from the same reserve unit, the Third Battalion, 25th Marines, based in Brook Park, Ohio, near Cleveland.

Haditha is one of a string of cities along the Euphrates River that American commanders believe forms the network that shuttles insurgents traveling through Syria into Baghdad and other parts of the Iraqi heartland.

The marines have begun a series of operations in recent months to bring the area under firmer control and to choke off the flow of insurgents. But success has been elusive.

The insurgent group Ansar al Sunna claimed responsibility for the ambush that killed the American snipers on Monday and said in an Internet posting on Wednesday that it had captured one of the marines alive. The group claimed that it had killed eight Americans altogether, and that its fighters had beheaded some of the Americans who were still alive after the ambush.

There was no way to verify the claim that the group had taken an American prisoner, and the posting did not contain any video or photographs. The group said a video of their prisoner was forthcoming.

Later in the day, however, Ansar al Sunna posted a video that seemed to depict the attack on the snipers. The video shows what appears to be two Humvees moving across the top of a desert ridge, and then, at different times, six distant figures walking in the desert off the road.

''These are the crusaders,'' a caption on the video says.

Then a voice can be heard chanting ''God is great!'' while three masked men firing mortar shells from atop a dune, though it does not show what they hit. The video shows what appears to be a dead American serviceman, his shirtless body burned and mangled.

The video offers a close-up of the man's lifeless face. Then a pair of hands reach down and cut the dog tag from around his neck. The camera zooms in on the tag, but it is not readable.

The closing frames of the video show an array of what appears to be captured American military equipment, including machine guns and M-40 sniper rifles with scopes.

''Pray for us,'' a caption says at the end.

At a news conference at the Pentagon, Brig. Gen. Carter Ham, a military spokesman, denied that any marines were being held by insurgents, saying that all of them had been accounted for. He said here was ''no indication'' that any of the marines killed on Monday had been beheaded.

American commanders did acknowledge that one of the marines caught in Monday's ambush had become separated from the others, and that his body was not found until later, about a mile away from the scene of the ambush. That suggested that it was at least possible that one of the marines had been captured alive by the insurgents and killed later. General Ham did not rule out that possibility.

''We just don't know what happened,'' General Ham said.