At least 11 fighters from Iraq's Popular Mobilization Forces were killed in an ambush by ISIS north of the capital on Saturday, PMF security sources said.
The militants used light weapons and the cover of darkness to target the PMF east of Tikrit, the capital of Iraq's Salahaddin province, two days after a twin suicide attack claimed by the group killed 32 people in Baghdad.
"ISIS launched an attack on PMF's Brigade 22," said one of the unit's officers Abu Ali al-Maliki.
Maliki told AFP the brigade commander was among those killed before reinforcements from the federal police came to the unit's aid.
PMF security sources said the total toll was 11 dead and 10 wounded.
There was no immediate claim of responsibility, but security sources interviewed by AFP blamed ISIS.
Iraq declared the group territorially defeated in late 2017, but has continued to battle extremist sleeper cells, mostly in the country's mountainous and desert areas.
Local troops have been aided by a US-led coalition, which first intervened to help fight ISIS in 2014 and continues to provide training, surveillance and airstrikes in support of anti-militant operations.
The coalition has significantly drawn down its troop numbers over the past year, with the US shrinking its force from 5,200 to 2,500.