BEIRUT — A U.S.-backed force fighting the Islamic State in Syria said Saturday that it had come under attack from forces supporting Syrian President Bashar Assad, escalating tensions on one of the country’s most complex and contested battlefields.

The Syrian Democratic Forces, or SDF, said the attack took place in the eastern province of Deir al-Zour, where the U.S.-backed militia is fighting the Islamic State as an Iran-backed force mounts a rival offensive.

“At 3am today, our forces East of Euphrates were targeted by Russian and Syrian regime warplanes in the Al-Sinaaiya area,” said the SDF military command in a statement, reporting that six fighters had been wounded.

It was not clear whether the casualties came from the airstrike or another altercation.

The Syrian Observatory for Human Rights confirmed the strike, but it did not elaborate on which pro-government force was responsible.

As the Islamic State’s territory shrinks across Syria and Iraq, Deir al-Zour province has become a hub for the group’s senior leaders. Stretching along the eastern border with Iraq, it has also become a geopolitical battleground for forces trying to support or thwart Iranian attempts to secure supply routes stretching from Beirut to Tehran.