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Russian warplanes harassed three American drones over Syria on Wednesday while they were taking part in a mission against jihadists, a US commander said.

"While three US MQ-9 drones were conducting a mission against ISIS targets, three Russian fighter jets began harassing the drones," Air Force Lieutenant General Alexus Grynkewich said in a statement, using an acronym for the Islamic State group.

The Russian jets dropped parachute flares in front of the US drones, forcing them to take evasive action, while one pilot turned on their plane's afterburner in front of an MQ-9, "reducing the operator's ability to safely operate the aircraft", he said.

"These events represent another example of unprofessional and unsafe actions by Russian air forces operating in Syria, which threaten the safety of both US and Russian forces," Grynkewich said, calling on Moscow to "cease this reckless behavior".

Earlier this year, the United States said a Russian jet clipped the propeller of a drone operating over the Black Sea, causing it to crash. Moscow denied it was responsible.