The U.S.-led coalition fighting ISIS says it hit pro-Syrian government forces near the border with Jordan that "posed a threat" to U.S. forces and rebel groups in the area.
The coalition statement late on Thursday says the coalition hit the pro-Syrian government forces after warning shots, a show of force and similar efforts by Russia failed to dissuade those troops from moving into the "de-conflicted area."
The de-conflicted zone, agreed between the United States and Russia, is located around Tanf, where the borders of Jordan, Syria and Iraq meet.
The statement says coalition forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against ISIS.
U.S. officials said earlier Thursday the strike had hit a tank and a bulldozer and forces there.
One official says the pro-regime forces loyal to Syrian President Bashar al-Assad were perceived as a threat to U.S.-allied troops there.
The officials weren't authorized to speak publicly on the matter and demanded anonymity.
Several Syrian opposition media groups with activists in the area say the airstrike hit a convoy of government troops and allied militiamen on the road to the Tanf area, where Syria's borders with Jordan and Iraq meet.
One opposition media group, the Palmyra News Network, says the attack at the Zarka juncture, about 27 kilometres from the border, destroyed a number of vehicles and led to casualties.
The area has been a source of tensions as both government forces and U.S.-backed rebels advance there. Both the government forces and the rebels are trying to force ISIS militants from the area.
The Revolutionary commandos, or Maghaweer al-Thawra, a U.S.-backed group, shared a report about the airstrike on their Twitter account.
The airstrikes come on the same day that UN-mediated talks got underway with a Syrian government delegation.
The head of the delegation said his team led the drive to start "informal meetings" on a constitutional process.
Bashar al-Ja'afari, who is also Syria's UN ambassador, said the subject was brought up on the government's "own initiative," but insisted that Damascus "will not accept any interference" on the constitution.
Thursday's announcement of the talks-within-talks among constitutional experts appear to be the only development so far in the sixth round of largely unproductive discussions that have been hosted by UN envoy Staffan de Mistura since early 2016.