Syria's state media said the USA -led coalition bombed government-backed troops in Khusham, a town in the oil-rich Deir el-Zour province, describing them as tribal fighters.
Regime forces operating Russian-made T-55 and T-72 tanks fired 20-30 tank rounds within 500 feet of the SDF base, where some USA troops were embedded, according to Pentagon press secretary Dana W. White.
"In defense of Coalition and partner forces, the Coalition conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition's defeat-Daesh mission".
The US-led coalition supported the Kurdish counteroffensive with artillery fire and air raids to prevent government forces from advancing in an area which was controlled by ISIS a few months ago.
Regime forces were likely attempting to take an oilfield near Khusham.
Although Turkey has backed opposition troops since the Syrian conflict began in 2011, Ankara has moved closer to Assad's ally, Moscow.
Meanwhile It is not clear what the makeup of the pro-Syrian government forces was and whether it included Syrian army soldiers.
The U.S. maintains about 2,000 troops in the country.
Also, the Russian Defense Ministry lashed out at the United States attack. "Pro-regime vehicles and personnel that were turned around and headed back west were not targeted".
In September 2014, the U.S. and Arab allies launch air strikes on the IS in Syria, opening a new front in the fight against the jihadist group, already targeted by raids in Iraq.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted PRF [pro-regime forces] attack", Veale said.
Ikhbariya described the groups it said had been bombed by the US -led coalition as "local people fighting [ISIS] and the SDF", Reuters reported. A pro-regime drone was also grounded after it dropped a munition in the vicinity of US troops in Syria. But that's been a challenging line for the U.S.to walk as human rights violations stack up against Assad and as his regime's forces threaten US troops and their allies.
Coalition warplanes and artillery hit the pro-Assad forces as they advanced across the Euphrates river into areas held by the Syrian Democratic Forces, the observatory said.
In June a year ago, a U.S. Navy jet shot down a Syrian plane after it attacked U.S. -backed fighters on the ground.