Moscow warned Washington Thursday that U.S.-supported forces will face a fierce military response if Russian-backed Syrian government troops come under fire again.

Russian military officials said they issued the statement after U.S.-backed Syrian Democratic Forces (SDF) troops twice fired on Russian-supported Syrian government troops fighting to oust Islamic State fighters from the oil-rich eastern province of Deir ez-Zour.

The U.S. military command “was told in no uncertain terms that any attempts to open fire from areas where SDF fighters are located would be quickly shut down,” Russian Defense Ministry spokesman Major-General Igor Konashenkov said, adding that any fire from the SDF would be “immediately suppressed with all military means.”

The SDF denied that it fired on Syrian government forces in the region, telling The Associated Press it was targeting ISIS targets instead.

Russia’s warning reflects the growing tensions between the U.S. and Russia as their rival proxies jockey for territory in their separate campaigns to drive ISIS from Deir ez-Zour, and seize the province’s valuable oilfields. Russian warplanes and special forces are backing Syrian army and allied militias as they advance on ISIS-held Deir ez-Zour from the south, while the U.S.-backed SDF, with the support of American special forces, is closing in on the terrorist group from the northwest.

In another sign of the rising stakes in the province, U.S. military officials revealed Thursday that American and Russian generals had held a rare face-to-face meeting this week to avoid accidental clashes between their proxies.