Five U.S. Army (search) soldiers participating in a combat operation were killed by a roadside bomb west of Baghdad during Iraq's constitutional referendum, the military said Sunday.

Saturday's explosion hit the soldiers' vehicle in the mostly Sunni Arab city of Ramadi (search), 70 miles west of Baghdad, where few Iraqis were believed to have voted in the referendum, the military said.

Though insurgent attacks were few in most of the country on voting day, Ramadi — a stronghold for insurgents — experienced violence. Six mortar rounds hit the Hall of Sports (search), a facility being used as a voting center, then clashes erupted between gunmen and U.S. troops in the city's center.

The five soldiers were assigned to the 2nd Brigade of the 2nd Marine Division. At least 1,975 members of the U.S. military have died since the war began in March 2003, according to an Associated Press count.