A U.S. official said late Wednesday that more than 100 forces loyal to Bashar Assad, were killed in the attack by the US-backed SDF in Deir Ezzor province. Syria state media accused Washington of a "war crime". Some 2,000 USA forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance, which holds the largest portion of Syria still outside the control of the government.
"Pro-regime forces initiated hostilities with artillery pieces (howitzers)".
Assad's strikes followed what the United States called an "unprovoked attack" by his forces on the headquarters of the Syrian Democratic Forces (SDF), a group of anti-Assad fighters the USA has trained and supported for years.
The US Defense Secretary Jim Mattis called the attack "perplexing" during a press conference on Thursday.
Damascus has also urged the United Nations to press the U.S. to leave, especially following the successful defeat of the main IS forces.
Violence continues to rage in Syria's civil war, which has killed hundreds of thousands of people since it began with a crackdown on protests in 2011.
"The Syrian government urged the UN Security Council to condemn the "'massacre"' and to hold the US-led coalition responsible for their "crimes and aggression". Coalition aircraft have struck regime forces who were perceived as posing a threat to a base at At Tanf, Syria, that houses USA and coalition military advisers. "To confront those who really fight global terrorism on the ground in Syria is criminal". "Beyond all doubt, it is an unprecedented act of aggression", said Frants Klintsevich, first deputy chairman of Moscow's upper house defense committee. The coalition said the strikes had been carried out to defend its allies. "Our forces have the inherent right to self-defense", she said. Nevertheless, the official said the USA is looking into whether Russian-affiliated forces may have been involved.
At least 19 civilians, including a medic and children, were killed in Syrian government strikes on opposition-held territory near the capital Damascus on Thursday, activists said. While Syria denies such claims, they are being investigated by the Organisation for the Prohibition of Chemical Weapons (OPCW). "ISIL is still there, and our mission is still to defeat ISIL", Pentagon spokeswoman Dana White said Thursday.
The U.S. -led coalition responded with "AC-130 gunships, F-15s, F-22s, Army Apache helicopter gunships and Marine Corps artillery", according to Fox News reporter Lucas Tomlinson.