The Russian warning underscored growing tensions over Syria between Moscow and Washington. While both oppose Islamic State (IS), they are engaged, via proxies, in a race for strategic influence and potential resources in the form of oilfields in eastern Syria's Deir al-Zor province.
The Russian Defense Ministry said the US-backed Syrian Democratic Forces (SDF) had taken up positions on the eastern banks of the Euphrates with US special forces, and twice had opened fire with mortars and artillery on Syrian troops who were working alongside Russian special forces.