Russian jets flew dangerously close to US drones over Syria on Thursday for the second time in less than 24 hours, according to the US military.

Lieutenant General Alex Grynkewich, head of US Air Forces Central Command, said the incident took place around 9:30 a.m. local time (0630 GMT/UTC) over northwest Syria on Thursday.

"Russian aircraft dropped flares in front of the drones and flew dangerously close, endangering the safety of all aircraft involved," Grynkewich said.

Colonel Michael Andrews, the spokesman for the US Air Forces Central Command, said "the Russian harassment, including close fly-bys, by one SU-34 and one SU-35 and deploying flares directly into the MQ-9, lasted almost an hour."

"So it wasn't a quick fly-by, but much more of a sustained and unprofessional interaction," he added.

Russian jets fly in front of American MQ-9 Reaper Wednesday, video shows

Russian jets also harassed US drones over Syria at about 10:30 a.m. local time a day earlier. The US Central Command released videos of the incidents on Wednesday and Thursday.

The video from Wednesday showed Russian SU-35 fighter closing in on an MQ-9 Reaper drone and one of the Russian pilots flying in front of it.

"One Russian pilot positioned their aircraft in front of an MQ-9 and engaged afterburner, thereby reducing the operator's ability to safely operate the aircraft," Grynkewich said of the first incident.

US asks Russia to cease 'reckless behavior'

Grynkewich said in a statement Thursday that "We urge Russian forces in Syria to cease this reckless behavior and adhere to the standards of behavior expected of a professional air force."

Both the US and Russia operate in Syria. The US has focused on fighting the so-called "Islamic State" militant group in Syria. Russia has been among President Bashar Assad's staunchest allies in the country.

The incidents come four months after a Russian jet forced down an American MQ-9 Reaper drone over international waters over the Black Sea.