US aircraft carried out rare, retaliatory strikes in Syria's Deir al-Zor province on Wednesday against forces loyal to Syrian President Bashar al-Assad after they attacked USA -backed fighters' headquarters there, US officials said.
In a statement, the coalition said its service members were located with the SDF, an alliance of militias in northern and eastern Syria, during an attack 8 km east of the Euphrates River deconfliction line.
The official says the strikes were in self-defense after the pro-government forces began firing artillery and tank rounds at the US-backed Syrian Democratic Forces.
One Syrian rebel soldier was wounded but no USA forces were killed or wounded, according to NPR's Tom Bowman, who is reporting from elsewhere in Syria.
Another US official who spoke to CNN said it was unclear exactly who the pro-regime forces were but that the US is looking into whether Russian contractors operating in the vicinity might have been involved.
"Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces headquarters February 7", the statement said.
The combination of air and artillery strikes came after pro-regime troops attacked the compound of the Syrian Democratic Forces.
"The US military is telling Damascus that it better think long and hard if it wants to go after America's partners", Mr Heras said.
He said the coalition suspected that pro-regime forces were attempting to seize territory, including lucrative oil fields, that the Syrian Democratic Forces had liberated from ISIS in September.
Neither U.S. officials nor the U.S. -backed coalition have offered details on the attacking forces. But the Trump administration's hope for a lessening of hostilities in Syria's larger civil war has not materialized so far, as Turkish, Russian, Iranian and other forces are brought into ever closer proximity as they vie to determine control of and pursue their own long-term interests in Syria. Any of the attacking vehicles and personnel who turned around and headed back west were not targeted, the military official said.
It's not the first time USA -led forces in the country primarily to hunt ISIS have targeted pro-Assad forces in self-defense.
The United States maintains about 2,000 troop in Syria, mostly working with the estimated 50,000 Syrian Democratic Forces. 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.