"Coalition struck Syrian pro-regime forces advancing in a de-confliction zone near al Tanf posing a threat to U.S. partner forces", it said.
US warplanes conducted airstrikes on forces believed to be loyal to Syrian President Bashar Assad in southern Syria on Thursday as a protection measure for USA forces based in the region, USA officials said.
The Syrian Observatory for Human Rights said the strikes killed eight people "most of them non-Syrian" and destroyed four vehicles.
The US defence official told AFP that the strike did not signal a strategic shift for the coalition, which has focused on battling IS and has tried to avoid intervening in Syria's civil war.
The US has been backing the Maghawir and Jaysh al-Thawra fighters in the near the Syria, Jordan, Iraq border.
Ominously, the Syrian regime has its own base fairly close to the one American forces are using to train opposition fighters, and the Syrian base is said to be ready to support "about a battalion's worth of troops". Still, the strike prompted debate among lawmakers about the USA role in the war and whether it was time for Congress to take up a new war authorization.
US -led troops helped repel a major IS attack on Tanf last month, while Russian warplanes bombed the base in June 2016. In Syria, Islamic State is fighting hard against several separate enemies, including Russian-backed Syrian government forces, trying to reinforce its presence as it is being pushed back in neighbouring parts of Iraq.More news: Dayton signs Real ID bill
Two US aircraft were dispatched as a "show of force" to get the Syrian vehicles to turn around.
"Such actions that were carried out against the Syrian armed forces. this is completely unacceptable".
The Syrian regime claimed the bombardment killed a number of people and said it revealed the "falseness" of the coalition's claim to be exclusively fighting Islamic State (IS) in the war-torn country.
This is the second time the United States has attacked directly forces of the Assad regime in Syria.
The UN-sponsored process has been somewhat overshadowed by a parallel track convened in the Kazakh capital, Astana, by regime supporters Russian Federation and Iran and rebel-backer Turkey.
The reported bombing came just days after the Treasury Department announced a new round of sanctions against Syrian companies and individuals on May 16, citing the Assad regime's "relentless attacks on civilians".