BAGHDAD, Iraq — A roadside bomb killed 10 Marines and wounded 11 others on a foot patrol near Fallujah, the U.S. military said Friday. It was the deadliest attack against American troops in four months.

The Marines from Regimental Combat Team 8, based at Camp Lejeune, N.C., were ambushed on Thursday. The unit has suffered some of the highest casualties of the Iraq war.

Also Friday, three U.S. soldiers from the 48th Brigade Combat Team were killed in a traffic accident south of Baghdad, and the military said an Army soldier assigned to the 2nd Marine Division died of wounds suffered the previous day when his vehicle was struck by a rocket in Ramadi, 70 miles west of the capital.

The military statement said seven of the wounded in Thursday's attack later returned to duty and that the rest of the team was conducting "counterinsurgency operations throughout Fallujah and the surrounding area" to improve security for the Dec. 15 elections.

The 14 deaths announced by the military Friday brought to at least 2,127 the number of U.S. service members who have died since the beginning of the war in 2003, according to an Associated Press count.

The attack in Fallujah came a day after President Bush outlined his strategy for victory in Iraq, and at a time when there are growing calls for an exit plan for U.S. troops.

Hours after the military announced its grim news, Al-Jazeera broadcast a videotape and statement in which the kidnappers of four Christian peace activists threatened to kill the hostages — two Canadians, an American and a Briton — unless all prisoners in U.S. and Iraqi detention centers are freed by Dec. 8.

U.S. forces have stepped up military operations throughout the Sunni Arab regions west of Baghdad to cut off the flow of weapons, ammunition and foreign fighters entering the country from Syria and to reduce insurgent activity.

As part of that campaign, the U.S. military on Friday launched a new offensive — Operation Shank — in Ramadi, capital of the insurgent-ridden Anbar province. About 200 Iraqi army soldiers and 300 U.S. Marines were taking part in the offensive.