The United States has conducted airstrikes against Iranian-backed paramilitaries in Iraq and Syria following the death of a US contractor.
Iraqi security sources said at least 25 fighters of the Kataib Hezbollah group were killed and more than 50 injured in Sunday's strikes.
The group is considered a terrorist organisation by the US.
A rocket attack on a base in Kirkuk on Friday killed a US contractor and wounded other personnel.
Speaking in Washington DC on Sunday, US Secretary of State Mike Pompeo warned Iran that the US would retaliate if American lives were threatened.
"Today what we did was take a decisive response that makes clear what President Trump has said for months - which is that we will not stand for the Islamic Republic of Iran taking actions that put American men and women in jeopardy," he said.
US Defence Secretary Mark Esper told reporters that F-15 jet fighters had hit five targets associated with Kataib Hezbollah. The Pentagon said three were in western Iraq and two in eastern Syria.