The Syrian government has said an aerial "aggression" on one of its military positions near the border with Jordan by the US-led coalition killed several soldiers.
The air strike was the first such confrontation between US forces and fighters backing Syrian president Bashar al-Assad and is likely to increase tensions in the war-torn country.
A Syrian military official confirmed the strike on state TV on Friday.
The US-led coalition announced on Thursday that it had launched an air strike targeting pro-Assad 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 (Isis).
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 military official said.
In Geneva, Bashar al-Ja’afari, the head of the Syrian government delegation to the peace talks under way in the Swiss city aimed at ending Syria’s six-year civil war, told reporters he had 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.
The US-led coalition has carried out a number of air strikes in recent months that have killed civilians in areas held by Isis.
Government offensive
The Syrian government, which has launched an offensive in the country's south backed by allied militias such as Iranian paramilitaries and Hizbullah, has vowed to take back the border area near Iraq.
That is likely to be opposed by the US and its allies, who back rebels fighting Isis in the same area.
A road between Iraq and Syria would facilitate further 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 had failed.
US officials and Syrian activists said the air 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 hit was a military position along the Tanf highway.
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.
AP