The airstrikes are understood to have hit a pro-regime militia that was advancing on US and coalition forces at At Tanf, according to Operation Inherent Resolve, the official name for the coalition fighting ISIS.
A USA military strike in Syria on Thursday was "government terrorism" and caused a massacre, Syrian government negotiator Bashar al-Ja'afari said on Friday.
The Syrian Observatory for Human Rights said the strike destroyed vehicles and killed eight militiamen.
"Such actions that were carried out against the Syrian armed forces... this is completely unacceptable, this is a breach of Syrian sovereignty", deputy foreign minister Gennady Gatilov was quoted as saying by state-run RIA Novosti in Geneva.
There were no immediate details on any civilian injuries.
The region around Tanf, where the borders of Jordan, Syria and Iraq meet, has been considered a de-conflicted zone, under an agreement between the USA and Russian Federation that went into effect earlier this month.
Coalition forces even reached out to Russian counterparts, via the standing deconfliction channel between the two militaries, in order to halt the Syrian troops advancing into the zone, command officials say.
Coalition forces, it went on to assert, had been operating near Al-Tanf "for many months, training and advising vetted partner forces" engaged in the fight against the Daesh terrorist group.
Last month, US-led allied forces and Syrian rebels thwarted a significant Daesh attack on the base. He was asked if the airstrike represented an increase in the USA role in the Syrian war.
More than 320,000 people have been killed in Syria since the conflict began following anti-government demonstrations in 2011.
It quotes another United States official as saying that a convoy of 20 pro-regime vehicles were headed toward At Tanf on Wednesday night, searching vehicles and buildings along the way.
Iran has started using the militias they support, including Hezbollah, a Lebanese-based group, to attack the SDF, said Ahmad Majidyar, an analyst at the Middle East Institute.
Rebel officials said earlier this week that hundreds of troops and militiamen were advancing into the sparsely-populated desert region, in an apparent attempt to halt recent rebel gains against so-called Islamic State (IS).