US-backed Syrian opposition factions will not let forces backing the regime of Syria’s Bashar al-Assad cross the Euphrates River in their bid to recover eastern Syria, their commander said.

Syrian opposition commander Abu Khawla said a civilian administration would be set up to run areas of Deir al-Zor province being captured from ISIS by his fighters, including its oil fields. The Syrian government was “not fit to lead and rule the people”, he said.

The Deir al-Zor military council, fighting as part of the US-backed Syrian Democratic Forces (SDF), has advanced toward Deir al-Zor from the eastern side of the Euphrates River since launching an offensive into the province a week ago.

Government forces, supported by the Russian air force and Iran-backed militias, have simultaneously advanced into Deir al-Zor from the west. Last week, they broke an ISIS siege of the provincial capital, Deir al-Zor city, which sits on the western bank of the river.

Deir al-Zor province is ISIS’s last major foothold in Syria and Iraq. Rich in oil, it is bisected by the Euphrates River and abuts Iraq.

The Russian- and US-backed campaigns against ISIS in Syria have mostly stayed out of each other’s way as the sides seek to avoid conflict, with the Euphrates often acting as a dividing line between the sides. Talks have been underway to extend a formal demarcation line that has separated the campaigns, officials have said.

Abu Khawla warned government forces and their militia allies against firing across the river as his fighters close in – something he said had happened in recent days.

“Now we have 3 km between us and the eastern riverbank, once our forces reach the area, any shot fired into that area we will consider an attack on the military council,” he said.