A Marine armoured vehicle, on patrol during combat operations near the Syrian border on Wednesday, hit a roadside bomb which resulted in 14 marines being killed.
The incident is one of the deadliest single attacks in Iraq against American forces.
A civilian interpreter also was killed in the bombing, which came two days after 7 marines died in the same area during combat with insurgents. At least 1,820 members of the United States military have died since the Iraq war began in March 2003, according to an Associated Press count.
Also, an American freelance journalist was found dead in the southern Iraqi city of Basra, the US embassy said. Steven Vincent had been shot multiple times after he and his Iraqi translator were abducted at gunpoint hours earlier, police said.
The Marines killed on Wednesday were assigned to the Regimental Combat Team 2, 2nd Marine Division, II Marine Expeditionary Force (Forward), the military said. One Marine also was wounded in the attack.
The incident occurred just outside Haditha, which is 140 miles northwest of Baghdad.
The names of those killed were not released.The latest losses come on the heels of the deaths of 7 US marines in combat 2 days ago in the volatile Euphrates Valley of western Iraq, where American forces are trying to seal a major border infiltration route for foreign fighters.
One of the Marines died in a suicide car bombing in Hit, 85 miles northwest of Baghdad. The other 6 were killed on Monday in Haditha, 50 miles from Hit, while battling insurgents. They all were attached to the same suburban Cleveland unit.
The American deaths come as the Bush administration is talking about handing more security responsibility to the Iraqis and drawing down forces next year.