The Pentagon said the US was supporting SDF forces following "an unprovoked attack by Syrian pro-regime forces".
The Syrian Observatory for Human Rights, meanwhile, said it had confirmed only 20 dead among the pro-government forces.
"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.
CNN also reported that HIMARS and MQ-9 drones were used in the attack.
Western powers have expressed alarm over the government's bombing campaign in rebel-held Eastern Ghouta, where 400,000 people have been living under siege since 2013.
"We know they were pro-regime forces", he said, "Iranian, Assad, Russian, mercenaries, I can't tell you". A spokesman for the US coalition said Russian Federation told the coalition that it was not involved in the attack on the SDF headquarters.
U.S. airstrikes in Deir Ezzor kills several Syrian troops
On Wednesday, US forces claimed that they attacked Syrian forces "in self-defense".
This came after militants in adjacent Idlib province downed a Russian fighter jet and killed the pilot on February 3, the AP said, citing unconfirmed Turkish media reports.
Now that the common goal of clearing out IS militants from Syria is nearly realized, Damascus and its allies have repeatedly threatened war against US troops if they don't leave the country.
"Nobody invited them there", Vasily Nebenzya stated, emphasizing that a hard fought for stability in Syria is being jeopardized by United States actions.
Meanwhile, Washington tried to ease tensions on Thursday, with Pentagon spokeswoman Dana White saying in a news briefing that the USA was not "looking for a conflict with the regime".
Reuters reported that regime forces were likely trying to take an oilfield near Khusham, citing a usa official.
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". This does, however, reflect the mission creep in Syria, as the defeat of ISIS has meant the U.S. forces there need a new justification to stay, and fighting the Assad government looks to be it, with the Pentagon already saying part of their pretext for staying is to ensure regime change.
Syria: Air defenses respond to Israeli strike near capital
The army has published a video of her destruction, as well as that of a vehicle in Syria participating in its implementation. At approximately 4.30 am, local time, Israeli forces sighted a drone being launched from a site within Syria.
Trump to sign US spending bill into law on Friday: White House
The effects from the overnight shutdown were expected to be minimal, but it will remain until Trump signs the funding plan. The Marist poll surveyed 1,012 US adults from February 5 to 7 and had a margin of error 3.7 percentage points.
The crude oil price moved closer to its lowest levels in 2018
Nationwide crude stockpiles in the previous week added 6.78 million barrels, the biggest gain in barrel terms in nearly 11 months. As prices have risen in the past year , production has jumped in non-OPEC countries such as Canada and the United States.