Militants ambushed a convoy of pro-government, Shi'ite Muslim paramilitary fighters in northern Iraq on Wednesday night, killing at least seven of them and wounding 30, Iraqi military and police said on Thursday.
No group immediately claimed responsibility, but Islamic State militants often carry out such attacks. The Sunni group has switched to insurgent hit-and-run tactics after losing almost all the territory it once controlled.
It has increased its attacks on the military, and such a high death toll especially among Shi'ite militias which were brought formally into the security forces last year is rare.
A military statement said Iraqi forces launched an operation pursuing "terrorist elements" who had attacked a group of fighters from the Popular Mobilization Forces - the umbrella group of Shi'ite militias - near the town of Makhmour.
Makhmour is located between Mosul and Kirkuk.
Iraqi forces backed by a US-led coalition captured in late 2017 all the territory that fell under Islamic State control in 2014 and 2015, including Mosul, which served as the militants' de facto capital.
The group's fighters have since waged a campaign of kidnapping, killing and bomb attacks targeting civilians and security forces.
Police sources said three buses carrying the militia fighters who were coming from Mosul came under attack by militants in a mountain area and was surrounded until military helicopters intervened to force the attackers to flee.
The fighters were heading on leave to Tuz Khurmatu, a town near the oil city of Kirkuk, police and militia officials said.
The dead and wounded were taken to hospitals in Mosul, police and health sources said.