The US says it has carried out air strikes against Iran-backed militias near the Iraq-Syria border in response to drone attacks on its forces in Iraq.
A Pentagon spokesman said the strikes targeted "operational and weapons storage facilities" at three locations.
President Biden had been clear he would act to protect US personnel, he added.
A powerful Iraqi militia alliance, the Popular Mobilisation Forces (PMF), said four members of one faction were killed and threatened to retaliate.
About 2,500 US troops are in Iraq as part of a global coalition supporting local security forces in their fight against the jihadist group Islamic State (IS).
There have been at least five drone attacks against facilities they use since April, US officials say. Rockets are also frequently fired at them.