A suicide bomber exploded a belt of explosives at a recruiting station for neighbourhood patrols in Iraq's Diyala province today, killing 13 volunteers and wounding 10, police said.
Police said US forces may have also been among the casualties in the attack in Kanaan, 20km southeast of Baquba, the province's capital north of Baghdad.
Mainly Sunni-Arab neighbourhood patrols have joined US forces to fight against al-Qaeda militants, one of the tactics Washington says has led to a 60 per cent drop in attacks in Iraq since June.
But the patrols have increasingly been targeted by militants, especially in provinces north of Baghdad, where US commanders say al-Qaeda has regrouped after being pushed out of other parts of Iraq.
US commanders said they had found a torture chamber in Diyala province with chains on the walls and a battery connected to an iron bed, describing it as proof of al-Qaeda's activity in the area.
One of the other main factors US commanders also credit for the decline in violence is a ceasefire by followers of the Shia Muslim cleric Moqtada al-Sadr.
Sadr's spokesman said the cleric was considering extending the six-month truce after it expires in February.