Russian fighter jets have again flown dangerously close to several US MQ-9 Reaper drones operating over Syria – the second such incident of harassment in 24 hours – setting off flares and forcing Washington’s unmanned aerial vehicles to take evasive manoeuvres, the United States air force said.

"We urge Russian forces in Syria to cease this reckless behaviour and adhere to the standards of behaviour expected of a professional air force so we can resume our focus on the enduring defeat of ISIS," Lieutenant General Alexus Grynkewich, head of US Air Forces Central Command, said in a statement.

The protest from the US air forces came as the French military said that two of its fighter jets on patrol over the Iraq-Syria border area were forced to manoeuvre “to control the risk of accident” involving a Russian Sukhoi SU-35 warplane on Thursday.

Earlier this year, the United States claimed that a Russian warplane had tripped over the thruster of a drone operating over the Black Sea and crashed. Moscow denied any responsibility.