A Russian warplane closely approached a US MC-12 reconnaissance aircraft over Syria, a US commander said Tuesday (July 18), describing it as an "unsafe" action that put the US crew at risk.

This was the latest in a series of interactions between Russian and US aircraft over Syria to draw criticism from the United States.

US forces are carrying out operations against the "Islamic State of Iraq and Syria" (ISIS) in Syria as part of the international coalition.

On Saturday, "a US MC-12... conducting operations in support of the coalition's Defeat ISIS mission was closely approached by a Russian Su-35 against established norms and protocols", Air Force Lt. Gen. Alexus Grynkewich said.

The US plane was forced "to fly through its wake turbulence", he added.

"This reduced the crew's ability to safely operate the aircraft and put the four crew members' lives at risk," he said, adding that this represents "a new level of unsafe and unprofessional actions by Russian aircraft operating in Syria".

On July 7, three MQ-9 drones were "harassed by Russian fighter aircraft while flying over Syria", Grynkewich said in a statement.

"During the almost two-hour encounter, Russian aircraft flew 18 unprofessional close passes that caused the MQ-9s to react to avoid unsafe situations," he said in a statement at the time.

Later the same day, the same US drones killed a top ISIS leader in a strike in eastern Syria, US Central Command said.

On July 6, Russian aircraft harassed US MQ-9 drones over Syria on two occasions within 24 hours, including by dropping flares in front of them and traveling "dangerously close", forcing US pilots to perform evasive manoeuvres.

And in March, 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.

During a Tuesday US State Department briefing, spokesman Matthew Miller noted that the United States and international coalition continue to work with local partners in Syria to maintain constant pressure on ISIS remnants.

Asked about increased Russian activity in Syrian skies, Miller reiterated previous Department of Defence statements on the matter.

"We strongly urge Russian forces in Syria to stop reckless and threatening behaviour that could result in an accident and loss of life, and adhere to the standards of behaviour expected of a professional force," he said.