Insurgents fired mortar rounds at an Iraqi National Guard headquarters in the city of Samarra today, killing four US soldiers and one Iraqi guardsman, the military said.
A fifth US soldier was unaccounted for and 20 others were wounded.
The attack destroyed a joint headquarters used by the Iraqi and US troops.
Samarra, 60 miles north of Baghdad, is in the so-called Sunni Triangle, a hotbed of anti-coalition resistance.
The city was wracked by violence throughout today.