(CNN) 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."

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