According to several sources, the recent air and artillery attack by the International Coalition forces in Deir Ezzor against the pro-government popular militias killed more than 100 people.
Syrian state-run media says the USA strikes killed and wounded dozens of tribal fighters, calling it a new effort "to support terrorism".
A USA 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.
In a letter addressed to the United Nations Secretary General, the foreign ministry said the attack "represents a war crime and a crime against humanity".
No U.S. military members were injured in the clash.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces", a USA military official said on condition of anonymity.The SDF and the coalition targeted the attacking forces with air strikes and shelling after "20 to 30 artillery and tank rounds landed within 500 metres (yards) of the SDF headquarters location", the official said.
Syrian ally Russian Federation has cast 11 vetoes on possible Security Council action on Syria since the civil war began in 2011, shielding Syrian President Bashar al-Assad's government.
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In the initial attack, Subedar Madan Lal Chowdhry, his daughter Neha, and Lance Naik Bhadur Singh were injured. The area was cordoned off after the arrival of the Army's special forces and the Special Operations Group.
"These are the worst four days that Eastern Ghouta has ever gone through", said Hamza, a doctor treating patients in the suburb of Arbin. The U.S.is looking into whether Russian contractors operating in the vicinity were also involved, according to another U.S. official who spoke to CNN, saying there is no direct evidence yet that the Russians had fired on the SDF facility.
The US-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.
Turkey was reported to have halted flights in recent days because Russian Federation had closed the airspace over the Kurdish stronghold of Afrin.
"I'm not sure why [regime forces] would do this, because it was [an SDF] headquarters", Mattis said, adding that the Russians told the United States military they did not have forces there.
One SDF fighter was wounded, officials said.
Meanwhile, Turkey's state-run Anadolu news agency said Turkish jets on Thursday targeted parts of Afrin, including Mount Bafilun, the villages of Sheik Huruz and Kefer Jenne and the regions of Sheran, Jinderes and Raju, the AP reported.