The bombing happened as soldiers and members of an anti-al Qaida security squad lined up to receive pay cheques in the town of Radwaniya.
The anti-al Qaida group is known as Sahwa or Sons of Iraq.
The Sahwa fighters have played a key role in the reduction of violence in Iraq since they first rose up against their al Qaida allies in late 2006, joining the US military and government forces in the fight against the terror group.
They have since been targeted by militants.
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