The US has carried out rare air strikes on Syrian pro-government forces after what it called an “unprovoked attack” on allied Kurdish and Arab fighters.
An estimated 100 pro-government fighters were killed in the overnight incident in Deir al-Zour province.
They had allegedly tried to take ground east of the River Euphrates, captured from the Islamic State group by the US-backed Syrian Democratic Forces (SDF).
State media said the US “aggression” left dozens of people dead or wounded.
The Middle Euphrates Valley serves as an informal demarcation line in eastern Syria, with the government controlling the western side and the SDF the east. The two sides have clashed there in the past year while seeking to drive IS militants from their last major stronghold in the country.
Elsewhere in Syria on Thursday, government warplanes bombed towns in the rebel-held Eastern Ghouta region near the capital Damascus for a fourth day.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said at least 36 civilians were killed, bringing the death toll to 185 since Monday.