US-backed Syrian militias said Russian warplanes struck their positions in Deir Ezzor province on Monday, generating further friction between two rival campaigns against ISIL.
The Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias fighting with a US-led coalition said the attack killed one of its fighters and injured two.
Major-General Igor Konashenkov of Russia's defence ministry denied the claim, saying Russia was always careful to ensure its air strikes were accurate.
A spokesman for the US-led coalition, Colonel Ryan Dillon, said rounds had hit in the area around the SDF but he could not confirm they were fired by Russia.
A war monitor, the Syrian Observatory for Human Rights, said Russian rounds had not hit the SDF near Deir Ezzor.
The SDF later said it had responded to the source of fire that targeted it from near the city of Deir Ezzor on Monday morning without specifying which forces it had attacked.
Russia and the United States back separate offensives against ISIL in eastern Syria, both of which are advancing in Deir Ezzor province, an area with large oil reserves bordering Iraq.
The assaults are converging on ISIL from opposite sides of the Euphrates that bisects the province, the extremists last major foothold in Syria, with the river often acting as a dividing line.
But their proximity has at times raised the risk of clashes that could stoke tensions between the competing world powers.
With Russian air support and Iran-backed militias, Syrian troops are advancing along the west bank of the river.
A Kurdish commander of the SDF, which has approached along the east bank with US jets and special forces, said the alliance is expected to completely push ISIL out of its former Syrian headquarters of Raqqa within a month.
With the Kurdish YPG militia at its forefront, the SDF said last week that, after seizing 80 per cent of Raqqa, the battle for the city had entered its final stages.
"In the coming days, the battles will be at their most intense...," said Jihan Sheikh Ahmed, spokeswoman for the offensive. "We expect, under our plan, that we will be able to liberate Raqqa in less than a month."
