Damascus - Syria has accused the US of carrying out a "brutal massacre" with a bombing attack in Deir al-Zour province.
The overnight air and artillery strikes killed an estimated 100 pro-government fighters near the Euphrates river, according to the US.
It claimed a right to self-defence, saying it was responding to an attack on its coalition forces. The Pentagon said Russian mercenaries were also killed, but Russia denies having personnel in the area.
The strikes happened in the Middle Euphrates Valley, which serves as an informal demarcation line in eastern Syria. The government controls the western side and the US-backed Syrian Democratic Forces (SDF) the east.
The two sides have clashed over the past year while trying to drive Islamic State (IS) militants from their last major stronghold in the country.
The US said the forces aligned with the Syrian government crossed the agreed line and launched an "unprovoked attack" on an SDF headquarters late on Wednesday, near the town of Khusham.
It said 500 pro-government fighters, backed by artillery, tanks, multiple-launch rocket systems and mortars, were involved in the assault.(FA)