BAGHDAD (AP) -- Islamic State militants ambushed a group of Iraq's Shiite-led paramilitary fighters, killing at least 27 more than two months after Baghdad declared victory over the extremist group, officials said Monday
The Popular Mobilization Forces, an umbrella group of mostly Shiite militias, said in a statement that the attack took place southwest of the northern city of Kirkuk, where the paramilitaries were conducting overnight raids.
The attackers were disguised in army uniforms and manning a fake checkpoint, the statement said, adding that ensuing clashes lasted for at least two hours and that some of the militants were killed while others fled the area.
Brig. Gen. Yahya Rasool, an Iraqi military spokesman, blamed IS "sleeper cells" and said Iraqi forces were searching the area to find the perpetrators.
IS claimed responsibility for the attack in a statement carried by its Aamaq news agency.
Karim al-Nouri, a PMF spokesman, described the attack as a "heinous crime" and called for greater scrutiny of Iraqis returning to areas liberated from IS. He said the attackers had taken advantage of heavy rains overnight.
Hadi al-Amiri, the leader of the Badr Brigade, one of the most prominent Shiite militias, vowed "revenge."
Speaking at a military airfield in Baghdad where the bodies were being flown in, he called on security forces to be vigilant, saying "the war against terrorism is not over yet."
Prime Minister Haider al-Abadi's office put out a statement expressing condolences to family members of those killed. It said it had issued orders to hunt down those responsible and other sleeper cells, and to investigate the incident and take any required steps.
At least 11 of the slain troops were from the southern city of Basra, where a three-day mourning period was declared.
Iraq declared victory over IS in December, after more than three years of heavy fighting. The group has been driven from all the territory it seized in the summer of 2014, but U.S. and Iraqi officials have said it is likely to continue launching insurgent-style attacks. Last month, IS launched back-to-back suicide bombings in central Baghdad, killing at least 38 people.
Associated Press writers Sinan Salaheddin in Baghdad and Maamoun Youssef in Cairo contributed to this report.