ISIS claimed responsibility for a deadly attack on a Shiite militia group near the Iraqi city of Hawija over the weekend, an ambush that left more than two dozen slain.
It was the deadliest attack against Iraq's Shiite-led Popular Mobilization Units since the country's forces wrested Hawija from the terror group in October.
At least 27 PMU fighters died in the Sunday night assault, southwest of Kirkuk, according to a statement released by the group on Monday.
Dozens of ISIS militants attacked several PMU military checkpoints on a highway near the town of Hawija, security officials in Kirkuk said.
The PMU, an independent military force of about 110,000 fighters, has been at the forefront of the fight against ISIS in Iraq. The group said it plans to "respond with determination to end the presence of ISIS criminal organization completely in the country."
Iraqi Prime Minister Haider al-Abadi ordered the punishment of the perpetrators and said the attack was carried out by "sleeper cells and terrorist pockets." His comments were made in a statement released by his office on Monday.
The Baghdad government in December declared victory over the terror group, announcing the end of three years of battles across the country. But the group is still able to carry out potent strikes.
ISIS claimed responsibility for a deadly attack last month in Baghdad.
At least 27 people were killed January 15 in a double suicide bombing in central Baghdad, according to the Iraqi Ministry of Health.
In addition to those killed, about 100 people were wounded when two bombers detonated suicide vests at a square in the Iraqi capital, officials said.