Gunmen dressed in military uniforms kidnapped eight former anti-Qaeda fighters from their homes north of Baghdad and killed them execution-style on Monday, officials said.
The murders come amid a series of attacks against the Sunni Arab tribal militia in recent days, and as Iraq grapples with a protracted political deadlock and months of protests by the Sunni community.
Analysts say the stalemate is likely to persist until general elections due next year.
The attack on the eight former anti-Qaeda fighters took place in Mishahada, which lies just north of Baghdad, when armed men kidnapped them during a pre-dawn raids on their homes.
Their corpses were later found in the town, having suffered gunshots to the head and chest, an interior ministry official and a medical source said.
The eight were all former fighters in the Sahwa, a collection of Sunni tribal militias that turned against Al-Qaeda and sided with the US military from late 2006 onwards, helping turn the tide of Iraq's bloody insurgency.
These anti-Qaeda militiamen are regarded as traitors by Sunni militants, and as a result are frequently targeted for attacks.
In the previous week alone, nine Sahwa fighters were killed and 18 others wounded in attacks, according to an AFP tally.
Iraq has seen a marked rise in violence since the start of the year, coinciding with protests by the country's Sunni Arab community, which says it is the target of discrimination at the hands of the Shiite-led authorities and security forces.
Analysts and diplomats say a failure by the government to address the underlying causes of the protests has given Sunni militant groups recruitment fodder and room to manoeuvre.