BEIRUT - Syrian troops crossed to the eastern side of the Euphrates river in Deir al-Zor on Monday, state media and a monitoring group reported, increasing their presence in an area where US-backed forces have also advanced.
A commander in a militia of the US-backed Syrian Democratic Forces (SDF) confirmed Syrian army units had crossed and said his fighters were ready to drive them back.
"If there are clashes between us and them - we're ready for those if the forces of the regime don't go back to the other bank," Ahmed Abu Khawla of the SDF's Deir al-Zor military council said.
Russia's foreign ministry said on Friday Syrian government forces crossed the river for the first time in their offensive against Islamic State in Deir al-Zor province.
Monday's crossing took place near a southern suburb of the city seized by the army on Saturday, the British-based Syrian Observatory for Human Rights said.
The convergence of the Syrian army offensive from the west and the separate SDF operation from the east has increased tension between the two sides.
The SDF said they came under attack from Syrian forces and Russian jets backing them on Saturday, after Abu Khawla said they would not allow the army to cross the river and would consider shots fired in their direction as an attack.
The SDF has advanced to within kilometers of the river, while the Syrian army has retaken much of the city, most of it on the western side.
The US- and Russian-backed offensives against Islamic State have generally 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.
President Bashar al-Assad has vowed to recapture all of Syria. Assad controls the main urban centers in the west of the country. The SDF, dominated by the Kurdish YPG militia, controls much of Syria's northeast. Syrian rebel groups hold pockets of territory in western Syria. Islamic State's shrinking territory includes much of oil-rich Deir al-Zor province.