NATO has expressed concern about the targets of Russia's air strikes in Syria on Wednesday.
The Kremlin said it hit IS militants, but this has been disputed by the United States and Western-backed rebels on the ground.
"I'm concerned about the reports saying that the Russian air strikes were not targeted against ISIL," NATO Secretary-General Jens Stoltenberg said during a visit to the United States.
"I'm especially concerned because there has been no real effort by the Russian side to deconflict the Russian air strikes in Syria with the ongoing US-led coalition fighting ISIL," he said.
Mr Stoltenberg urged Russia to play a "constructive and cooperative" part in the fight against IS, adding that "supporting Assad will not help".
Incursions by Russian jets into Turkish airspace have prompted a stern response from Nato's most powerful ally.
Here's what you need to know about this latest development in the increasingly bloody, complicated and volatile conflict.
Russia urges world leaders to unite to fight terrorism in Syria amid criticism that air strikes have targetted rebel fighters and civilians.