The attack, which occurred Thursday and left 11 other Americans wounded, took place while the Marines were conducting anti-insurgency operations in Western Iraq.
The U.S. military announced the deaths Friday in a statement that said Marines "continue to conduct counterinsurgency operations throughout Fallujah and surrounding areas to provide a secure environment for the national elections, Dec. 15."
Insurgents detonated the bomb, which was apparently constructed of several large artillery shells, as members of the Regimental Combat Team 8 of the 2nd Marine Division patrolled on foot some 30 miles west of Baghdad.
The attack was the deadliest against American forces since 14 Marine Reserve troops from Ohio were killed in August when their amphibious assault vehicle was blown up by a roadside bomb.
The bombing is only the latest news of American casualties stemming from anti-insurgent operations ahead of elections. Also on Thursday, a U.S. Army soldier died in a rocket attack in Ramadi and four American service members were killed Wednesday, three of them from hostile action and the fourth in a traffic accident.
According to the Associated Press, at least 2,121 members of the U.S. military have died since the beginning of the war in 2003.
But the deaths also came just days after President Bush outlined a new strategy for achieving political and military victory in Iraq.
In Washington, White House spokesman Scott McClellan said President Bush had been told of the deadly attack Thursday night and then more fully briefed about the incident Friday morning.
"We are saddened by the loss of life, whether it's one soldier who loses his or her life, or 10 or 11, we are saddened to hear that news," McClellan said.
"Our heart and prayers go out to the families, their loved ones paid the ultimate sacrifice for an important cause and we are forever grateful for their service and sacrifice," he added.
According to AP, the combat team hit Thursday is part of the II Marine Expeditionary Force based at Camp Lejeune, N.C. The unit has been one of the hardest hit in the combat in Iraq. Since the war began in 2003, 147 Marines from II MEF have died in combat, according to 2nd Marine Division spokesman Lt. Barry Edwards.