Russian aircraft once again harassed US MQ-9 Reaper drones over Syria Friday, the Air Force said, in a sign of increasing friction between the two countries in Middle East airspace.

The incident marked the third time this week that US drones over Syria were intercepted by Russian aircraft.

"Earlier today three MQ-9 drones were once again harassed by Russian fighter aircraft while flying over Syria," commander of US Air Forces Central Lt. Gen. Alex Grynkewich said in a news release. "During the almost two hour encounter, Russian aircraft flew 18 unprofessional close passes that caused the MQ-9s to react to avoid unsafe situations."

"We continue to encourage Russia to return to the established norms of a professional Air Force so we can all return our focus to ensuring the enduring defeat of ISIS," Grynkewich added.

On Thursday, Russian fighter jets harassed a US MQ-9 Reaper drone that was conducting a mission against ISIS targets in northwest Syria. One of the Russian jets dropped flares in front of US drone in an apparent attempt to hit the drone, forcing it to take evasive maneuvers, the Air Force previously said.

And earlier in the week, three Russian jets dropped parachute flares in front of three US drones, forcing the drones to take evasive maneuvers. One Russian jet also lit its afterburner in front of a US drone, limiting the drone operator's ability to safely operate the aircraft.

Russia is operating in Syria in support of Syrian President Bashar al-Assad, while the US maintains its presence as part of the anti-ISIS coalition.

While the two countries have used a deconfliction line in Syria over the last several years to avoid unintentional mistakes or encounters that can inadvertently lead to escalation, Russian military actions have increasingly violated the deconfliction protocols, including flying too close to US military bases in Syria.

But the US wasn't the only target of harassment from the Russian military this week. On Thursday, a Russian SU-35 fighter jet conducted a "non-professional interaction" with two French Rafale fighter jets that were flying a mission near the Iraq-Syria border, according to the official Twitter account of the French Armed Forces.