The Pentagon says it carried out military strikes in Iraq and Syria, targeting a militia group.
Kataib Hezbollah, backed by Iran, is blamed for an attack last week that killed a U.S. civilian contractor at an Iraqi military base.
U.S. officials say Sunday's (December 29) strikes were retaliation.
Official images showed what the Pentagon says were militia targets, struck by U.S. forces.
Aides to President Donald Trump called the operation successful.
Secretary of State Mike Pompeo spoke after briefing Trump.
(SOUNDBITE) (English) U.S. SECRETARY OF STATE MIKE POMPEO, SAYING: "This has been going on now for weeks and weeks and weeks.
This wasn't the first set of attacks against this particular Iraqi facility and others, where there are American lives at risk.
And today what we did was take a decisive response that makes clear what President Trump has said for months and months and months, which is that we will not stand for the Islamic Republic of Iran to take actions that put American men and women in jeopardy." And defense chief Mark Esper says F-15s were launched against five targets.
(SOUNDBITE) (English) U.S. SECRETARY OF DEFENSE MARK ESPER, SAYING: "The targets we attacked included three targets in western Iraq and two targets in eastern Syria that were either command and control facilities or weapons caches for Kataib Hezbollah." Esper also warned further U.S. action was on the table.
Iraqi security and militia sources said at least 25 militia fighters were killed in three U.S. air strikes Sunday.
The sources say four Kataib Hezbollah commanders were among the dead.