U.S.-backed Syrian militias said Russian jets and Syrian government forces struck their positions in Deir al-Zor province on Monday, killing and injuring a number of fighters.
There was no immediate comment from Moscow or Damascus.
The Syrian Democratic Forces, an alliance of Kurdish and Arab militias, said they came under attack near a major gas field they had seized from Islamic State in recent days.
“We will not stand by with our arms crossed and we will use our legitimate right to self-defence,” said the statement by the SDF, which fights alongside the U.S.-led coalition.