The Pentagon said the us was supporting SDF forces following "an unprovoked attack by Syrian pro-regime forces".
"Syrian pro-regime forces included approximately 500 personnel in a large, dismounted formation supported by artillery, tanks, multiple-launch rocket systems, and mortars", a USA military official said, according to Radio Free Europe.
The attack was weaned.
Russian Federation says a recent United States attack on pro-Damascus forces proves that Washington is not fighting Daesh but rather it is seeking to plunder Syria's economic assets.
The Euphrates River, flowing through Northeastern Syria, serves as a demarcation line between the USA backed SDF and Russian Federation and Iran backed pro regime forces believed to be active around the city of Deir el-Zour.
Nevertheless, he declined to answer questions on whether there were any new contacts between the Russian and U.S. militaries in connection with the shelling of Syrian pro-government forces by the coalition near the inhabited community of Khusham.
One hundred and twenty of those who were granted amnesty entered the ranks of the US -backed SDF umbrella group, according to Nebenzia. Some 2,000 USA special forces troops remain on the ground in Syria, allied to the Kurdish-led SDF alliance, which holds the largest swathe of territory still outside the control of the government.
Syria's official SANA news agency described the USA "aggression" as an "attempt to support terrorism".
Overall, the pro-government force included about 500 people supported by rocket and mortar systems and Soviet-era tanks, the official said.
The rare strikes against pro-regime forces were confirmed by the US-led coalition Operation Inherent Resolve, which is in the region targeting Islamic State.
Elsewhere in Syria, U.S. -led airstrikes killed 100 Syrian government fighters, the Pentagon said.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted (enemy) attack", the official said.
"We are not looking for a conflict with the regime", she added.
No U.S. troops were killed or wounded in the incident, officials said. The offensive was described by US Army Col. Thomas Veale, a spokesman for the US-led coalition, as a coordinated attack against the USA -backed forces.
The US said it was responding to an "unprovoked attack" on allied Kurdish and Arab fighters.
The Syrian Observatory said they were local tribesmen loyal to President Bashar al-Assad and Afghan Shia militiamen. "It's not always readily apparent" what sort of forces they are maneuvering on the ground in Syria's crowded battlespace, he said.
Russian Foreign Ministry spokeswoman Maria Zakharova said Thursday that military experts and diplomats were now collecting data on the incident and would comment later. The chaos unleashed in the original rebellion to topple Mr Assad gave room for jihadist groups such as Islamic State, as well as the Kurdish forces that are now USA partners trying to establish their own enclaves.