The US-led coalition killed more than 100 Assad regime fighters in eastern Syria on Wednesday, officials said, in the largest deliberate strike carried out by Western forces against pro-Syrian government troops.
The coalition said it struck regime fighters with airstrikes and artillery after they launched an "unprovoked attack" against the Syrian Democratic Forces (SDF), the Western-backed rebels fighting the Islamic State of Iraq and the Levant (Isil) in Syria.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces," a US military official said.
The strikes took place in the Euphrates River valley in the province of Deir Ezzor, where Isil still holds scraps of territory but is under intense pressure from both Syrian regime forces and the SDF.
The US has struck pro-regime forces in eastern Syria several times but Wednesday's attack caused more casualties than any previous deliberate strike.
Around 100 Syrian army troops were killed in an accidental strike in Deir Ezzor in September 2016 when US forces mistook them for Isil fighters.
The US and the Assad regime have for the most part maintained an uneasy truce in Syria, exchanging hostile words but rarely shooting at each other
Wednesday's strike against pro-regime forces illustrates the complex new reality in Syria, where the shared enemy of Isil has largely been defeated and tensions are growing between competing military forces in the country.
The US said that around 500 pro-regime troops, backed by tanks and artillery, had launched what appeared to be coordinated attack on an SDF headquarters.
“The coalition conducted strikes against attacking forces to repel the act of aggression,” a coalition spokesman said.
The US did not specifically say if the fighters were from the Syrian army or an allied militia or a foreign fighters group like the Lebanese militants Hizbollah.
The Syrian regime said the fighters were from a local militia fighting against Isil and accused the US of "a war crime and a crime against humanity".
Damascus regularly protests against the US-led intervention in Syria, calling it a violation of Syria's sovereignty, but has been largely powerless to stop it.
The US maintains a "deconfliction channel" with Russia, which is allied with the Syrian regime, and the two militaries use the channel to coordinate their movements to avoid to any accidental conflicts.
The US said it had been in contact with the Russians throughout the attack on Wednesday and had been reassured that Russian aircraft would not launch strikes against the coalition in support of Syrian regime troops.
Further strikes on regime positions are being considered within the US government, a Syrian opposition diplomatic source told the Telegraph, particularly within the National Security Council led by General HR McMaster, and supported by Nikki Haley, the ambassador to the UN.
But there is significant divergence within the administration, according to the source, with Secretary of State Rex Tillerson urging more caution.
The French government is said to support such moves, having been frustrated by what it saw as flip-flopping within the Trump administration over their willingness to stand up to Russia in Syria.