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.
The US has designated Kataib Hezbollah a terrorist organisation since 2009
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.
Pentagon spokesman Jonathan Hoffman said US forces conducted “precision defensive strikes” against three Kataib Hezbollah facilities in Iraq and two in Syria that would degrade its ability to conduct attacks against US-led coalition forces based in Iraq to fight the Islamic State (IS) group.
These locations included weapon storage facilities and command and control locations that Kataib Hezbollah used to plan and execute attacks, he added.
A spokesman for the Popular Mobilisation, an Iraqi paramilitary force dominated by Iran-backed Shia militias like Kataib Hezbollah, said the US strikes had targeted its Al-Jazira Operations Command facility in the al-Qaim area of Anbar province, as well as fighters from its 45th and 46th brigades.
US Defence Secretary Mark Esper told reporters the strikes were “successful” and that he had discussed with President Donald Trump “other options” to discourage further attacks on US forces.
“I would note also that we will take additional actions as necessary to ensure that we act in our own self-defence and we deter further bad behaviour from militia groups or from Iran,” he said.