A US drone strike has killed an Islamic State group leader in Syria after Russian warplanes harassed MQ-9 drones over the war-torn country, the US Central Command said Sunday.

The strike on Friday resulted in the death of Osama al-Muhajer, IS leader in eastern Syria, Centcom said in a statement.

"We have made it clear that we remain committed to the defeat of ISIS throughout the region," Centcom chief General Michael Kurilla was quoted as saying, using another acronym for the IS jihadist group.

"ISIS remains a threat, not only to the region but well beyond," he added.

According to Centcom, no civilians were killed in the operation but coalition forces are "assessing reports of a civilian injury".

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

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

Air Force Lieutenant General Alexus Grynkewich said the planes "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".

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

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 rebel-held Idlib province.

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