The US military has carried out an air strike targeting Iran-backed militias in Syria, the Pentagon says.
The attack destroyed "multiple facilities located at a border control point used by a number of Iranian-backed militant groups", it said.
It was the first military action ordered by the Biden administration and came in response to attacks against US and coalition personnel in Iraq.
A civilian contractor was killed in a rocket attack on US targets this month.
A US service member and five other contractors were also injured when the rockets hit sites in Irbil, including a base used by the US-led coalition, on 15 February.
Rockets have also struck US bases in Baghdad, including the Green Zone, which houses the US embassy and other diplomatic missions.
The Pentagon named Kataib Hezbollah and Kataib Sayyid al-Shuhada as two of the Iran-backed militias it targeted in Thursday's strike near the Iraqi border in eastern Syria.
It called the action a "proportionate military response" that was taken "together with diplomatic measures", including consulting coalition partners.
The operation "sends an unambiguous message", the Pentagon statement said.
"President Biden will act to protect American and Coalition personnel. At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation in both eastern Syria and Iraq."
'We know what we hit'
The US did not confirm any casualties, but UK-based monitoring group, the Syrian Observatory for Human Rights, said the US attack had killed at least 17 pro-Iranian fighters.
"The strikes destroyed three lorries carrying munitions," the observatory's Rami Abdul Rahman told AFP. "There were many casualties."
Kataib Hezbollah has denied any role in the recent rocket attacks, but US Defence Secretary Lloyd Austin told reporters he was "confident in the target that we went after".
"We know what we hit," he said. "We're confident that that target was being used by the same Shia militants that conducted the strikes" on 15 February.
An unnamed US official told Reuters news agency that the attack was meant to send a signal that while the US wanted to punish the militias, it did not want it to escalate into a bigger conflict.