At least 22 people were killed when a suicide bomber blew up himself among a group of pro-government, anti-Islamic State group Sunni militias near Baghdad, Iraqi officials said.
The attacker managed to mingle with militiamen who had gathered at a military base in the town of Madain to receive their monthly payment, two police officers said.
At least 15 of the dead were Sunni militiamen and the rest were soldiers, while 55 other people were wounded.
The town is about 14 miles (20km) south of Baghdad.
The group, known as Sahwa or the Awakening Councils, is made up of Sunni militiamen who joined US troops in the fight against al Qaida during the height of Iraq’s insurgency in 2007 and 2008. They are viewed as traitors by the Sunni militants.
Three medical officials confirmed the casualty figures.
No-one immediately claimed responsibility for the bombing, but it bore the hallmark of the Islamic State group, which captured large swathes of western and northern Iraq in a summer blitz.
The militant group’s onslaught has become Iraq’s worst crisis since the 2011 withdrawal of US troops.