Twenty-seven members of Hashd al-Shaabi, or the Popular Mobilization Forces (PMF), were killed after clashes broke out late Sunday in Hawija, a town in Iraq’s oil-rich Kirkuk province, the militia said in a statement on Monday.
ISIL – also known as ISIS – claimed responsibility for the attack and said 30 people had been killed in the ambush, local media reported.
It was the deadliest attack against PMF fighters since October when pro-government forces retook Hawija from ISIL.
The killings raise questions about the continued presence of ISIL fighters in Iraq after Baghdad declared victory in its fight against the armed group in December.
The PMF, an Iran-backed paramilitary group aligned with the Iraqi government in its battle against ISIL, said the attackers had been wearing Iraqi army fatigues and were manning a fake checkpoint.
PMF soldiers were conducting security operations around Hawija at the time of the attack, the militia said.
A senior police officer in the province, who asked not to be named, told AFP news agency most of the bodies had been beheaded.
Iraq recaptured vast expanses of territory from ISIL fighters last year, including the major cities of Mosul, Ramadi and Fallujah.
Last week, international donors pledged $30bn to help the Iraqi government rebuild the country in the aftermath of the four-year battle against ISIL.
Homes, schools and hospitals across Iraq have been destroyed in the fighting, and the country is struggling to rebuild its shattered infrastructure and economy.