Rare clashes broke out Sunday between Syrian regime forces and a US-backed alliance in the east of the country, killing six alliance fighters, the Syrian Observatory for Human Rights monitor said.
State news agency SANA said the army seized control of four villages from the US-backed Syrian Democratic Forces in Deir Ezzor province, where the Kurdish-led alliance has been fighting the Islamic State jihadist group.
The SDF and Russian-backed Syrian government forces are conducting parallel but separate offensives against IS in the oil-rich province.
But both sides have largely avoided running into each other and a deconfliction line exists to avoid such incidents.
The Kurdish-led alliance is present on the eastern side of the Euphrates River that cuts across Deir Ezzor province, while regime forces hold territory on the river's western bank, including the provincial capital of the same name.
"The regime's aim is to protect Deir Ezzor city by pushing back SDF fighters from the eastern bank" of the Euphrates, Observatory head Rami Abdel Rahman said.
Incidents between both sides have occurred in the past.
In February, the US-led coalition backing the SDF carried out air strikes in Deir Ezzor that killed at least 100 pro-regime fighters.
Washington said the strikes were in retaliation for an attack on its own personnel and SDF forces.
In September last year, the SDF accused Russia of killing one of its fighters in strikes on a gas facility newly retaken from IS.
A car drives past a damaged mosque in the eastern Syrian city of Deir Ezzor in this November 5, 2017 file photo