Syrian has said an aerial "aggression" by the US-led coalition on a government military position near the border with Jordan killed several soldiers and caused material damage.
The air strike was the first such close confrontation between US forces and fighters backing President Bashar Assad and the development is likely to increase tensions in the war-torn country.
A Syrian military official confirmed the strike on state TV a day after the strike.
The US-led coalition said on Thursday that an air strike hit pro-Syrian government forces it said posed a threat to American troops and allied rebels operating near the border with Jordan.
The attack comes at a time when the US presence in Syria is becoming more visible, mostly in northern parts of the country where American troops are backing Kurdish-led forces fighting Islamic State.
This "flagrant aggression launched by the international coalition exposes the falsity of its allegation about fighting terrorism and undoubtedly demonstrates the reality of the Zionist-American project in the region", the Syrian official said.
In Geneva, Bashar al-Ja'afari, the head of the Syrian delegation to peace talks under way in the Swiss city, told reporters he discussed the air strike during a meeting with UN special envoy Staffan de Mistura, describing the attack as "state terrorism".
"In every meeting we remind the attendees that there is terrorism by terrorists and also terrorism by states committed against our country," Mr al-Ja'afari said.
He was referring not only to Thursday's strike but also to others carried out by the US-led coalition in recent months that have killed civilians in areas held by IS.
The Syrian government, which has launched an offensive in the country's south backed by allied militias such as Iranian paramilitaries and the Lebanese Hezbollah group, has vowed to reach the border area with Iraq.
That is likely to be opposed by the US and its allies who back rebels fighting IS in the same area. A road between Iraq and Syria would facilitate the flow of assistance from Iran, which is a strong backer of Assad.
The coalition said "apparent" Russian attempts to stop pro-Assad forces from moving toward Tanf, as well as warning shots and a show of force, had failed.
US officials and Syrian activists said the strike took place in the desert near the border with Jordan, although it was unclear if it struck the Syrian army or just militias allied with the government.
The Syrian military official said the area attacked was a military position along the Tanf highway in the desert.
The area around Tanf, where the borders of Jordan, Syria and Iraq meet, has been considered a de-conflicted zone, under an agreement between the US and Russia that came into effect earlier this month.