Five fighters from the Kurdish-led Syrian Democratic Forces were killed in an overnight attack on an American base in eastern Syria, a war monitor said Monday.
"Five members from SDF special forces were killed and around 20 wounded by a drone attack after midnight on Al-Omar oil field," the largest US-led coalition base in Syria, said Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
The Islamic Resistance in Iraq, a loose alliance of pro-Iran fighters opposed to US support for Israel in Gaza, claimed a drone attack on Sunday "against the US occupation base in the Al-Omar oil field".
The Observatory's Abdel Rahman said the attack hit an SDF section inside the base, in "the first attack by pro-Iran groups against American bases after the US strikes on Syria and Iraq" late last week.
On January 28, a drone slammed into a base in Jordan, killing three US soldiers and wounding more than 40, an attack Washington blamed on Iran-backed forces.
The US responded Friday with a series of unilateral strikes on Iran-linked targets in Syria and Iraq, and has said it would press on with its retaliation.
The Observatory, which relies on a network of sources inside Syria, had said at least 29 pro-Iran fighters were killed in the US strikes in Syria.
A US-led coalition was set up in 2014 to fight Islamic State group jihadists who had seized swathes of Iraq and neighbouring Syria.
Roughly 2,500 US troops are deployed in Iraq and about 900 in Syria as part of the coalition.