Iraqi police officials say the attack took place Saturday afternoon when a suicide bomber set off his explosives belt at a gathering of the militiamen in the town of Taji. The blast also wounded 17 people.
Taji is 20 kilometers north of Baghdad.
Medical officials confirmed the casualty figures from the attack. All officials spoke on condition of anonymity as they were not authorized to release the information.
Thousands of Iraqi Shiites answered a call from clergy in June to join security forces fighting the Sunni militants, who have captured large swaths of land in the country's west and north.