Syrian regime troops cross the Euphrates near US-backed SDF
Syrian regime forces crossed to the eastern side of the Euphrates in Deir az-Zour on Monday, increasing their presence in an area where US-backed forces have made advances.
The rival forces are both conducting operations against the Islamic State group but have generally avoided each other, with the river often acting as a dividing line.
This year, however, incidents have raised tensions between the Syrian regime, backed by Russia, and US-backed forces.
Talks have been taking place to extend an official demarcation line, officials had said.
A military commander for the US-backed Syrian Democratic Forces (SDF) – an alliance of Kurdish and Arab fighters – confirmed Syrian regime forces had crossed and his fighters were ready to fight 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 az-Zour military council said, according to Reuters.
Monday's crossing reportedly took place near a southern suburb of the city seized by the regime on Saturday, the British-based Syrian Observatory for Human Rights said.
Parallel operations by the Syrian regime from the west of the city and the SDF from the east have led to increased tensions.
Russia has denied that it was responsible for the airstrikes.
The Syrian regime has recaptured most of the city, the majority from its western side, while the SDF has advanced to within kilometres of the river.
The SDF controls much of Syria's northeast.