BEIRUT: U.S.-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 U.S.-led coalition, said the strikes wounded six of its fighters.
Washington and Moscow support 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 the Syrian government of trying to obstruct its fighters. 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 Moscow or Damascus.
The assaults by the Russian-backed Syrian army and the U.S.-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 U.S. 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.
The U.S. coalition has said the SDF does not plan to enter Deir al-Zor city, where the Syrian army recently broke an Islamic State siege that lasted three years.
Still, in June, the SDF accused the Syrian military of bombing its positions in Raqqa province and the United States shot down a government warplane in Syria's crowded airspace.
ACROSS THE RIVER
Ahmed Abu Khawla, the commander of the SDF's Deir al-Zor military council, said Russian or Syrian fighter jets flew in from government territory before dawn on Saturday.
The warplanes struck as the SDF waged "heated and bloody battles" in the industrial zone on the eastern bank, seizing factories from Islamic State militants, he said.
"We have requested explanations from the Russian government," he told Reuters. "We have asked for explanations from the coalition ... and necessary action to stop these jets."
The Syrian Observatory for Human Rights, a Britain-based war monitor, said coalition planes had pounded the same industrial zone overnight to help SDF advances against Islamic State.
The strikes came a day after Khawla said his fighters would not let Syrian government forces cross the Euphrates.
On Friday, he warned the army and its allies against firing across the river, something he said they had done in recent days.
The Russian foreign ministry said units of the Syrian army had already crossed.
A senior aide to President Bashar al-Assad said the government would fight any force, including the U.S.-backed militias, to recapture the entire country.
"I'm not saying this will happen tomorrow ... but this is the strategic intent," Bouthaina Shaaban said in a TV interview.
Shi'ite militias fighting with Damascus launched attacks on Saturday against Islamic State in the south of Deir al-Zor province along the border. Just over the frontier inside Iraq, the military said Iraqi armed forces dislodged Islamic State from the natural gas-rich Akashat region.
Islamic State's declaration in 2014 of a "caliphate" spanning both countries effectively collapsed in July, when an Iraqi offensive captured Mosul, the militants' capital in Iraq.
(Reporting by Ellen Francis; editing by Robin Pomeroy and Clelia Oziel)