The US has conducted air strikes in Iraq and Syria against an Iran-backed Iraqi militia blamed for an attack that killed an American civilian contractor.
Weapons caches and command and control centres at five sites associated with Kataib Hezbollah were hit on Sunday, the defence department said.
An Iraqi paramilitary force said 25 fighters were killed and 51 injured.
The US secretary of state said it would not stand for Iran taking actions that put American lives in jeopardy.
Kataib Hezbollah leader Jamal Jaafar Ibrahimi, who is also known as Abu Mahdi al-Muhandis, warned that “the blood of the martyrs will not be in vain”.
“Our response will be very tough on the American forces in Iraq,” he said.Iraq’s prime minister denounced the “violation of Iraqi sovereignty”, while Iran’s foreign ministry said the US strikes were a “clear example of terrorism”.
The American contractor died when more than 30 rockets hit an Iraqi military base in the northern city of Kirkuk on Friday. Four US service personnel and two members of the Iraqi security forces were also wounded in the attack.