U.S.-backed Syrian militias said Russian warplanes struck their positions in Deir el-Zour province on Monday near a natural gas field they seized from Islamic State last week, but Russia denied it.
The Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias fighting with a U.S.-led coalition said the attack killed one of its fighters and injured two others.
Russia's RIA news agency cited Major-General Igor Konashenkov as issuing the denial and saying Russia was always careful to ensure its airstrikes were accurate.
A spokesman for the U.S.-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.
Russia and the United States back separate offensives against Islamic State in eastern Syria, with both advancing in oil-rich Deir el-Zour bordering Iraq.
The assaults are converging on Islamic State from opposite sides of the Euphrates river that bisects Deir el-Zour, the jihadists' last major foothold in Syria.
With Russian air power and Iran-backed militias, Syrian troops closed in from the west.
A Kurdish commander of the SDF, which has advanced from the east with U.S. jets and special forces, told Reuters on Monday the alliance expected to completely push Islamic State out of its former Syrian headquarters of Raqqa in less than a month.
The SDF also captured the large gas field on the Euphrates' bank on Saturday.
"Russian and regime forces have mounted a treacherous attack against our forces [there] ...with artillery and aircraft," the SDF said in a statement.
"We will not stand by with our arms crossed and we will use our legitimate right to self-defense."
The Russian- and U.S.-led forces battling Islamic State in Syria have mostly stayed out of each other's way, with the river often acting as a dividing line. But the proximity of their offensives has at times raised the prospect of clashes that could stoke tensions between the competing world powers.
Last week, the Pentagon accused Moscow of bombing SDF positions on the eastern side of the river. Russia denied this, warning the United States it would target the SDF if its own forces came under fire.
Dillon, spokesman for the U.S.-led coalition, said it was communicating with Russia to ensure measures to avoid unintended conflict between the offensives were being followed.