US-backed militias said they came under attack on Saturday from Russian jets and Syrian government forces in Deir al-Zor province, a flashpoint in an increasingly complex battlefield.
The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias fighting with the US-led military coalition, said the strikes wounded six of its fighters.
Washington and Moscow are backing separate offensives in the Syrian conflict – with both sides advancing against Islamic State militants in the eastern region that borders Iraq.
“Our forces east of the Euphrates were hit with an attack from the Russian aircraft and Syrian regime forces, targeting our units in the industrial zone,” the SDF said in a statement.
The SDF accused Damascus of trying to obstruct its battle against Islamic State. Such attacks “waste energies that should be used against terrorism … and open the door to side conflicts,” it said.
There was no immediate comment from the Syrian government or Moscow.
The assaults by the Russian-backed Syrian army and the US-backed SDF have at times raised fears of clashes that could stoke tensions between the competing world powers.
The offensives have converged on Islamic State from opposite sides of the Euphrates river, which bisects oil-rich Deir al-Zor, Islamic State’s last major foothold in Syria.
Syrian troops with Iran-backed militias have closed in from the west since last week, while the SDF advances from the east.
Russian and US battles against Islamic State in Syria have mostly stayed out of each other’s way, with the Euphrates often acting as a dividing line. Talks have been under way to extend a formal demarcation line, officials have said.
In June, the SDF accused the Syrian army of bombing its positions in Raqqa province and the United States shot down a Syrian government warplane.