The incident occurred in Mashada, 20 kilometres (12 miles) north of Baghdad, and, although the motive was unclear, a police officer based in nearby Tarmiyah said the victims may have been the targets of an ‘honour killing.’

‘Some of the men killed were members of the Sahwa (Awakening),’ a militia founded by the US army, said the officer, who gave the toll on condition of anonymity.

The Sahwa, also known as ‘The Sons of Iraq’ by American forces, are Sunni tribesmen who joined forces with the US military in 2006 to combat the fierce sectarian insurgency that had brought the country to the brink of civil war.

The movement is widely credited with turning the tide of the battle against Al Qaeda. Sahwa members have also been singled out by insurgents as being legitimate targets.

Tarmiyah was once a stronghold of Al Qaeda but fighters were later pushed back by members of the Awakening.

The latest attack follows a similar incident just over a week ago in a village near Baghdad that saw 13 members of a tribe headed by a Sahwa leader gunned down.

Earlier on Wednesday, two bombs within minutes of each other exploded at a restaurant in the southern shrine city of Karbala, wounding at least 26 people, police and medical officials said.

The first bomb targeted diners inside the restaurant in the heart of the city around 9:00 am (0600 GMT), causing several injuries, police said.

However, a second bomb minutes later, after an ambulance and medics had arrived to help the wounded, caused most of the casualties.