At least one Syrian soldier was killed and seven others wounded in a US-led coalition strike in central Syria late Thursday, the Syrian Observatory for Human Rights said.
The Britain-based monitor said the bombing raid targeted a Syrian government position in a desert area in the central province of Homs.
"There was a vehicle moving around that gathering point, which lies just 20 kilometres away from Al-Tanf," the Observatory's director Rami Abdel Rahman told AFP.
"The vehicle and the position were both targeted," he told AFP. One Syrian soldier was killed and another seven were wounded, according to Abdel Rahman.
Al-Tanf is a garrison located along Syria's southeastern border with Jordan and is used by US-led coalition forces to train Syrian rebels to fight the Islamic State group.
But there are Syrian government troops and allied fighters from Iran, Iraq and Lebanon positioned near the zone.
To keep them from interfering with the anti-IS efforts, the coalition has agreed with regime ally Russia to a 55-kilometre de-confliction zone around Al-Tanf.
The US has carried out raids against regime forces and its allies if it found they were getting too close to the zone.
On Thursday, the Pentagon told AFP there was a brief clash but denied it had carried out a strike and said there were no casualties.
Spokesman Major Adrian Rankine-Galloway said fighters from the US-backed Syrian rebel group Maghawir al-Thawra and coalition advisors "within the de-confliction zone near al-Tanf were engaged by an unidentified hostile force located just outside the de-confliction zone".
The rebels and coalition advisors "returned fire in self-defence, at which time the hostile force disengaged", Rankine-Galloway said.
The incident came after a deadly air strike on Sunday evening on the border town of Al-Hari in eastern Syria, near the frontier with Iraq.
Fifty-five fighters including Syrians and Iraqis were killed. A US official speaking anonymously in Washington blamed Israel.
The raid was particularly deadly for the Iraqi fighters working alongside the Syrian regime. Iraq's powerful Hashed al-Shaabi paramilitary force said 22 of its members were among those killed.
Placed under the authority of the Iraqi premier, these forces include groups close to Iran, including the Hezbollah Brigades, which lost fighters in the strikes.
In recent months, Israel has stepped up its military incursions against arch-foe Iran's forces in Syria.
On May 24, 12 pro-regime fighters died in an air raid against Syrian army positions south of Abu Kamal, a town a few kilometres from the Iraq border.
The Syrian Observatory and Syrian state media had attributed the strike to the coalition although this was denied by the Pentagon.
On February 7, the coalition admitted to killing at least 100 pro-regime fighters in the eastern Deir Ezzor province, including Russians.
It has been bombing IS in both Syria and neighbouring Iraq since 2014.