Baghdad, Iraq ? The U.S. military on Friday announced the deaths of 10 Marines and the wounding of 11 others near the western city of Fallujah in the single deadliest attack on U.S. troops in Iraq in four months.

A huge roadside bomb fashioned from artillery shells hit the Marines as they patrolled on foot on the city’s outskirts Thursday, the military said, and the attack was remarkable not only for its size but because it occurred in an area that U.S. commanders had considered well under control.

The Pentagon withheld the names of the dead pending notification of their families but said all were members of Regimental Combat Team 8, 2nd Marine Division, 2nd Marine Expeditionary Force.

Seven of the wounded have returned to duty, the military said. The severity of the injuries of the other four Marines was unclear.

The grim news came as U.S. and Iraqi forces began an anti-insurgent operation involving 500 troops in Ramadi in western Anbar province, and as kidnappers holding four Western peace activists threatened to kill them unless all U.S. and Iraqi detainees are freed.

The military also announced Friday that four U.S. troops had died elsewhere in Iraq, pushing the number of American military personnel killed in the conflict to at least 2,124, according to an Associated Press count.

The attack Thursday near Fallujah was the most lethal against U.S. forces since a roadside bomb killed 14 Marines aboard a personnel carrier Aug. 3 near Haditha in western Iraq.

The latest military deaths come at a difficult time for the Bush administration as the president finds himself reeling from the lowest poll numbers since taking office and as American disapproval of the war has spiked. On Wednesday, Bush made a major speech at the U.S. Naval Academy in which he outlined his plan for victory in Iraq.

The administration also has been buffeted with questions about its handling of the war.

The most recent controversy concerns reports that a U.S. contractor has been paying Iraqi journalists to publish positive stories about the U.S. military. On Friday, Sen. John Warner, R-Va., chairman of the Armed Services Committee, met with Pentagon officials to discuss the issue.

Meanwhile, the U.S. military announced Friday that U.S. and Iraqi troops had launched an offensive in the turbulent western city of Ramadi.

About 300 Marines and 200 Iraqi troops are taking part in the offensive, dubbed Operation Shank, targeting terrorist operations in the city 70 miles west of Baghdad.

The operation comes on the heels of reports from mostly Arab media outlets that insurgents had taken control of some areas in Ramadi.

The U.S. military said that those reports were inaccurate and that the city was largely quiet. On Friday, however, the military reported that a U.S. soldier assigned to the 2nd Marine Division, 2nd Expeditionary Force died of injuries he suffered a day earlier when a vehicle he was traveling in was struck by a rocket.

In a separate development, the satellite news channel Al-Jazeera broadcast a new videotape in which the captors of four Western peace activists who were kidnapped late last month threatened to kill them unless all prisoners in U.S. and Iraqi detention centers are released.

Al-Jazeera reported that a note attached to videotape said that the captors gave Dec. 8 as a deadline for their demands to be met. The kidnappers are holding an American, a Briton and two Canadians from the Chicago-based Christian Peacemaker Teams.