The Pentagon said the USA was supporting SDF forces following "an unprovoked attack by Syrian pro-regime forces".
The U.S. -led coalition in Syria has killed an estimated 100 pro-Assad fighters in air and artillery raids this week, including military contractors from Russian Federation, reports say.
The strikes killed 73 people on Thursday, according to SOHR.
"The coalition conducted strikes against attacking forces to repel the act of aggression" against its own personnel and the Kurdish-led Syrian Democratic Forces (SDF) hosting them, it said.
According to the coalition, around 500 pro-regime soldiers attacked the Syrian Democratic forces using artillery, mortar, and Russian-made tanks.
Russian officials assured coalition officials they would not engage coalition forces in the vicinity, she said.
Damascus called the attack a war crime, while the Russian military asserted that Washington's true goal is to capture "economic assets" in Syria. New details are emerging about the incident, suggesting both that the Syrian attack was far overstated, and the United States retaliation was overwhelmingly excessive.
The strikes inflicted what could be the largest number casualties that the coalition has exacted against the pro-regime forces.
According to Igor Girkin, an ex-commander of Russia-backed separatists in eastern Ukraine, two tactical units of the Wagner private military group had been hit by the USA strikes on February 7.
Moscow has reminded Washington that its presence in Syria is illegal after the US-led coalition struck pro-government popular forces.
The US authorities added that the Syrian government forces organized and executed an attack on the Syrian Democratic forces without any form of provocation near the Khusham town on Wednesday this week.
According to Russia's Ministry of Defense, the advancing pro-regime forces had not coordinated with Russian officials in the area. "The incident was caused by uncoordinated reconnaissance and search operations plans of the Syrian militias with the Russian command task force in the locality of Salhiyah".
Islamic State fighters may indeed be on the run - for which the poor beleaguered people of Syria are grateful - but the fighting over valuable territory and oil rights lingers on.