Russian air strikes have targeted US-backed Syrian Democratic Forces and coalition advisers east of the Euphrates River in Syria's Deir Az Zor province, according to the Pentagon.
Saturday's incident wounded several SDF fighters, but the coalition advisers were unharmed, it said in a statement.
"Russian munitions impacted a location known to the Russians to contain Syrian Democratic Forces and coalition advisers," the Pentagon said.
Russia's military spokesman earlier denied targeting SDF, an alliance of Arab and Kurdish fighters.
"This is not possible. Why would we bomb them?" Igor Konashenkov, Russia's military spokesman, told AFP news agency at the Hmeimim base, Moscow's main outpost for its air operations in Syria.
The attack was first reported by the SDF in a statement, which accused the Syrian government - backed by Russia air force - of trying to obstruct its fighters as both of the forces battle the Islamic State of Iraq and the Levant in oil-rich Deir Az Zor province - ISIL's last major foothold in Syria.
Such attacks "waste energies that should be used against terrorism ... and open the door to side conflicts", it said.
On Friday, Bouthaina Shaaban, top adviser to Syrian President Bashar al-Assad, told Al-Manar television that it was up to Russia and the US to see that the SDF and Syria's army do not clash.
"Whether it's the SDF, Daesh, or any illegitimate foreign forces ... we will work against them until all our land is liberated," she told the channel, which is run by allied Lebanese movement Hezbollah, referring to ISIL using its Arabic acronym Daesh.
"It's the role of Russia and America to see that this confrontation doesn't take place," she said.