US drones that had been harassed by Russian aircraft earlier in the day on Friday (July 7) killed a top "Islamic State of Iraq and Syria" (ISIS) leader in eastern Syria, US Central Command (CENTCOM) said Sunday.

CENTCOM said it conducted a strike on Friday that resulted in the death of Usamah al-Muhajir, an ISIS leader in eastern Syria.

The strike was conducted by the same MQ-9 Reaper drones that Russian aircraft had harassed the same day in an encounter that had lasted almost two hours.

A defence official told ABC News that al-Muhajir was killed while riding a motorcycle in northwestern Syria in an area where the US has carried out other strikes against terrorist leaders.

US drones taking part in operations against ISIS in Syria also were harassed Wednesday and Thursday by Russian military aircraft.

Air Force Lt. Gen. Alexus Grynkewich said the Russian planes on Thursday "dropped flares in front of the drones and flew dangerously close, endangering the safety of all aircraft involved".

Russian planes also flew dangerously close to French Rafale fighter jets.

Three Russian jets on Wednesday dropped parachute flares in front of US drones, forcing them to take evasive action, Grynkewich said, calling on Moscow to "cease this reckless behaviour".

Russian warplanes also fired thermal grenades on the vicinity of al-Bab and Bazaa in eastern Aleppo province during Russian manoeuvres in the region, the Syrian Observatory for Human Rights said Wednesday.

This caused an outbreak of fire in the area, the Observatory said.

The recent incidents are just the latest examples of an uptick in aggressive and unprofessional behaviour from Russian pilots in Syrian airspace that have obstructed US and international coalition anti-ISIS operations.

Committed to defeat of ISIS

"We have made it clear that we remain committed to the defeat of ISIS throughout the region," said CENTCOM commander Gen. Michael "Erik" Kurilla.

"ISIS remains a threat, not only to the region but well beyond."

During the month of June, US and international coalition partners conducted 37 operations against ISIS targets in Iraq and Syria, resulting in the death of 13 ISIS operatives and the detention of 21, CENTCOM said July 6.

"The continued reduction in ISIS activities in Iraq and Syria [is] a testament to our partner forces' commitment to the safety and stability of the region," said international coalition commander US Maj. Gen. Matthew McFarlane.

"The coalition continues to advise, assist and enable our partners in the lead toward an enduring defeat of ISIS," he said. "These operations showcase our enduring commitment to the lasting defeat of ISIS to preserve regional security and stability."

"We commend the competence, professionalism and dedication of our Iraqi, Syrian, and coalition partner forces," Kurilla said.

"Our continued efforts to defeat ISIS can only be accomplished through sustained and comprehensive partnered operations."