Russia warned the United States of reprisals Thursday after artillery fire from an area controlled by US-backed fighters in eastern Syria targeted government troops.

A military spokesman in Moscow blamed Syrian Democratic Forces and said future attacks would be repelled.

"Syrian regime forces were twice targeted with massive fire from mortar launchers and rocket artillery from areas east of the Euphrates river where SDF forces and US special forces are," Igor Konashenkov said in a statement.

"A representative of US command in Al Udeid, Qatar, was notified in severe form that attempts to attack from districts where SDF fighters are located will be repelled," he said.

"Fire positions in these areas will be immediately suppressed with all available weapons," he said.

Russia is supporting the current offensive by Syrian government forces against Islamic State jihadists near the city of Deir Ezzor with air cover, while Russian special forces are also active in the area.

Konashenkov said regime forces control 85 percent of Deir Ezzor. "The city will be liberated in the coming week," he said.

On Monday, Syrian government troops crossed the Euphrates river, sealing off the city on three sides. 

Meanwhile, SDF, an alliance of Kurdish and Arab fighters, is pushing the IS group from the northeastern part of the province, as the US-led coalition provides air cover.

To prevent the two operations from clashing, the coalition, the SDF, Syria's government and Russia agreed on a "de-confliction line" that runs from the neighbouring province of Raqa southeast along the Euphrates to Deir Ezzor.

But on Saturday the US-led coalition and the SDF accused Russian warplanes of bombing SDF fighters east of the river, an allegation that Moscow denied.

Konashenkov said Syrian regime forces have continued the offensive on the eastern side of the Euphrates over the past day, "freeing 16 square kilometres and two villages from terrorists."

A longtime ally of Syria, Russia stepped into the multi-front conflict in September 2015 and shifted the balance of power in favour of President Bashar al-Assad with extensive air support and other aid.