A suicide bomber detonated an explosives-rigged vehicle at a security forces position north of Baghdad on Friday, killing 15 people, police and a doctor said.
The attack south of the sensitive shrine city of Samarra in Salaheddin province, where militants have overrun the state capital and a swathe of other territory, also wounded 25 people.
Samarra, 15 kilometres (nine miles) north of where the attack took place, is home to the revered Shiite Al-Askari shrine, which was bombed in February 2006, sparking a bloody Sunni-Shiite sectarian war.
The position was occupied by a mix of Iraqi soldiers and civilians who have volunteered to fight a major jihadist-led militant offensive that has overrun chunks of five provinces.
Security forces folded under the weight of the initial onslaught, in some cases shedding uniforms and abandoning vehicles to flee, prompting the government to announce it would arm and equip people who volunteered.
While government forces have since performed better, they are still struggling to carry out effective offensive operations against the militants.