By Ellen Francis

BEIRUT (Reuters) – 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 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 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.

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.

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-held territory before dawn.

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 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 at SDF positions across the river – which 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.

The U.S.-led coalition said last week that the SDF did not plan to enter Deir al-Zor city, where Syrian troops recently broke an Islamic State siege that had lasted three years.

A pro-Damascus military alliance launched attacks on Saturday from the southern corner of Deir al-Zor province to drive Islamic State from the Iraqi border.

Islamic State is also coming under attack by U.S.-backed Iraqi government forces just over the border from Syria’s Deir al-Zor inside Iraq.

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 Andrew Heavens and Robin Pomeroy)