US airstrikes killed 11 pro-Iran fighters in eastern Syria following a drone attack that killed one American contractor and wounded five US service personnel, a war monitor said on Friday.
Defense Secretary Lloyd Austin said on Thursday that at the direction of President Joe Biden, he had authorised "precision airstrikes tonight in eastern Syria against facilities used by groups affiliated with Iran's Islamic Revolutionary Guard Corps".
The IRGC is a wing of the Iranian military and is blacklisted as a terrorist group by the United States.
"The airstrikes were conducted in response to today's attack as well as a series of recent attacks against Coalition forces in Syria by groups affiliated with the IRGC," Austin added.
A Department of Defense statement said the US contractor had been killed and the others wounded "after a one-way unmanned aerial vehicle struck a maintenance facility on a Coalition base near Hasakeh in northeast Syria".
Another US contractor was also injured in the UAV attack, the Pentagon said, adding that the US intelligence community "assess the UAV to be of Iranian origin".
On Friday, Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a Britain-based monitor with a wide network of sources on the ground in the war-torn country, said 11 people had been killed by US strikes.
"US strikes targeted a weapons depots inside Deir Ezzor city, killing six pro-Iran fighters, and two other fighters were killed by strikes targeting the desert of Mayadine and near Albu Kamal," he said.
Hundreds of US troops are in Syria as part of a coalition fighting against remnants of the Islamic State (IS) group and have frequently been targeted in attacks by militia groups.
The US troops support the Syrian Democratic Forces (SDF), the Kurds' de facto army in the area, which led the battle that dislodged IS from the last scraps of their Syrian territory in 2019.
Two of the US service members wounded on Thursday were treated on site, while the three other troops and one US contractor were medically evacuated to Iraq, the Pentagon said.
"We will always take all necessary measures to defend our people and will always respond at a time and place of our choosing," said General Michael Kurilla, commander of US Central Command.
When the strikes were announced, Biden had already travelled to Canada, where he is set to meet with Prime Minister Justin Trudeau.
Last August, Biden ordered similar retaliatory strikes in the oil-rich Syrian province of Deir Ezzor after several drones targeted a coalition outpost, without causing any casualties.
That attack came the same day that Iranian state media announced a Revolutionary Guard general had been killed days earlier while "on a mission in Syria as a military adviser".
Iran says it has deployed its forces in Syria at the invitation of Damascus and only as advisers.
Meanwhile, Russian jets have been regularly flying over US air bases in March, in violation of deconfliction agreements between Washington and Moscow. The deconfliction was implemented four years ago to ensure that no direct confrontation takes place, despite being on opposing sides.
Russian jets have flown approximately 25 times over US air bases this month, as opposed to zero times in February.