At least 19 Iraqis, 18 of them National Guardsmen, have been killed in a car bomb explosion, the US military says.
Iraqi security forces have been repeatedly targeted in the area
A suicide bomber set off the explosion next to an Iraqi security force bus near the town of Balad, north of Baghdad, a US military spokesman said.
The attack happened in the area known as the Sunni Triangle, which has been the scene of repeated violence.
Insurgents have staged a series of attacks in the run-up to nationwide elections planned for 30 January.
The BBC's Jim Muir in Baghdad says this is one of the most deadly incidents in the recent spate of attacks, which are increasingly directed at Iraqis as well as American troops.
Iraqi police and National Guards will be providing security during the elections.