The attack took place in the village of Halaiwat, near the disputed town of Tuz Khurmatu, at about 2:00 am (2300 GMT on Thursday), said a police officer.
Gunmen dressed in military uniforms gathered the militiamen -- known as Sahwa, or Awakening -- from the area then shot them, the officer said, adding that one survived but was in critical condition.
A doctor from Tuz Khurmatu hospital confirmed the toll.
The Sahwa are made up of Sunni Arab tribesmen who joined forces with the US military against Al-Qaeda from late 2006, helping turn the tide against the insurgency.
Sunni militants still linked to Al-Qaeda regularly target Sahwa fighters in because they regard them as traitors.
The Iraqi government announced at the end of January that about 41,000 Sahwa fighters are to receive 500,000 Iraqi dinars ($415) a month, up from 300,000 dinars ($250).
An increase in wages for the Sahwa, as well as their incorporation into the security forces and civil service, has long been a demand of Iraq's Sunni community.