Syria has accused the US-led coalition of "war crimes" after air raids in oil-rich Deir Az Zor province targeted its fighters.
A US official told Reuters news agency on Thursday that more than 100 pro-government fighters were killed in the overnight raids.
The US said they had responded to an "unprovoked attack" against the Washington-backed Syrian Democratic Forces (SDF), but did not give any details on casualties.
A Pentagon spokesperson told reporters that the SDF had "acted in self-defence with support" from the US-led coalition.
"Pro-regime forces initiated what appeared to be a coordinated attack on Syrian Democratic Forces east of the Euphrates river deconfliction line," Dana White said.
"Syrian Democratic Forces supported by the coalition targeted the aggressors with a combination of air and artillery strikes," she added.
Syrian news agency SANA disputed the US account and said that the US-led coalition had "attacked popular forces that were fighting" armed groups such as the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).
The Syrian government described the air raids as "a new aggression that poses a war crime and a crime against humanity".
It called on the UN Security Council to condemn the "massacre", according to state news agency SANA.
Russia, an ally of Assad, said the goal of the US military presence in Syria is "aimed at taking control of the country's economic assets and not at fighting against [ISIL]", Russian news agency TASS reported.
The SDF is a US-backed alliance of Kurdish and Arab fighters led by the Kurdish Peoples' Protection Units (YPG).
"The area in which the attack took place is significant not just because it's one of the places known to have oil within Syria, but more importantly because who controls it on the ground is the Syrian Democratic Forces," Al Jazeera's Jamal Elshayyal, reporting from Gaziantep, on the Turkish side of the Syria-Turkey border, said.
"Washington depended heavily upon them in the fight against ISIL over the years and now find themselves in a situation where it's pegged against Turkey and others who accuse this militia of essentially being just as bad as ISIL themselves.
The air raids coincided with escalating violence elsewhere in Syria.
More than 200 people were killed in the Damascus suburb of Eastern Ghouta and in Idlib, while Turkey continues a military offensive to clear the YPG from the northwestern border town of Afrin.