A drone strike by the United States has reportedly killed a Daesh leader, Usamah al Muhajir, in eastern Syria.

No civilians were killed in the operation on Friday, but coalition forces are "assessing reports of a civilian injury," the US military said in a statement on Sunday.

"We have made it clear that we remain committed to the defeat of Daesh throughout the region," the Central Command chief was quoted as saying.

"Daesh remains a threat, not only to the region but well beyond," the statement added.

The US army claims that Friday's strike "was conducted by the same MQ-9s (drones) that had... been harassed by Russian aircraft in an encounter that had lasted almost two hours."

A US commander said at the time that US drones taking part in operations against Daesh in Syria were harassed on Thursday, for the second time in 24 hours, by Russian military aircraft.

Efforts to combat Daesh

Russia is a key ally of the Syrian regime of Bashar al Assad.

Air Force Lieutenant General Alexus Grynkewich said the planes on Thursday "dropped flares in front of the drones and flew dangerously close, endangering the safety of all aircraft involved".

In another incident on Wednesday, three Russian jets dropped parachute flares in front of US drones, forcing them to take evasive action, Grynkewich has said, calling on Moscow to "cease this reckless behaviour".

With the support of Moscow as well as Iran, Assad has clawed back much of the ground lost in the early stages of the Syrian conflict that erupted in 2011 when the government brutally repressed pro-democracy protests.

The last pockets of armed opposition to the regime include large swathes of the northern Idlib province.

The United States has about 1,000 troops deployed in Syria as part of international efforts to combat Daesh, who were defeated in Syria in 2019 but still maintain hideouts in remote desert areas and conduct frequent attacks.