The United States has accused Russian pilots of putting both aircraft and crewmembers at risk with a series of unsafe and unprofessional maneuvers this month over Syria, where Moscow's forces are deployed to support President Bashar al-Assad.

On Sunday, "Russian fighter aircraft flew dangerously close to a US MQ-9 drone on a defeat-ISIS mission, harassing the MQ-9 and deploying flares from a position directly overhead," US Air Force Lieutenant General Alexus Grynkewich said in a statement, using an acronym for the Islamic State group.

"One of the Russian flares struck the US MQ-9, severely damaging its propeller," but it was able to return to its base, Grynkewich said, calling on "Russian forces in Syria to put an immediate end to this reckless, unprovoked, and unprofessional behavior."

While most of the recent incidents have involved harassment of drones, Grynkewich said a Russian jet "closely approached" a manned reconnaissance plane on July 16, forcing it to fly through the warplane's wake turbulence and reducing the "crew's ability to safely operate the aircraft."

Earlier in the month, the United States said Russian aircraft harassed American MQ-9 drones over Syria on two occasions within 24 hours, including by dropping flares in front of them.

And in March, Washington said a Russian jet clipped the propeller of an MQ-9 drone operating over the Black Sea, causing it to crash.