Security forces killed an ISIS commander and four fighters in the Hamrin Mountains area of north-east Iraq on Sunday, the military said.
Iraqi and US-led coalition warplanes have been carrying out air strikes on militant hideouts in the area for three days.
The elite Counter Terrorism Service and the military's operations command for Diyala province did not identify the commander but said in a statement he was in charge of ISIS in Hamrin.
He and four followers were killed in the area north-east of Baquba, the provincial capital, on Sunday.
Iraq declared victory over the group, which once held large areas of the country, in December 2017.
Having also been defeated in Syria, ISIS has switched to hit-and-run attacks aimed at undermining the Baghdad government.
The militants regrouped in the Hamrin range in the north-east, which extends from Diyala province on the border with Iran north-west to the Tigris River in Kirkuk province.