An air strike near a US base in southeastern Syria has killed at least eight pro-government fighters, the Syrian Observatory for Human Rights reported Monday.
Four Syrians, one Iranian national and three other non-Syrian fighters were killed in the strike carried out on Saturday, the Britain-based war monitor said.
At least 11 people were wounded in the attack, according to Observatory figures.
"A convoy of Iranian forces and allied militia was hit by air strikes as it drove near Al-Tanf base," the monitor's head Rami Abdel Rahman said.
He could not confirm the strike had been conducted by the US-led coalition present in the region.
A coalition spokesman told AFP the Al-Tanf base "received fire from unknown forces, with no damage and the coalition forces did not fire back".
Several strikes against Syrian government or allied forces have in the past been attributed to US forces, which were deployed with the declared goal of fighting the Islamic State (IS) group.
The base, set up in 2016 near the borders with Iraq and Jordan, was also used for the training of so-called "vetted opposition" to the regime of President Bashar al-Assad.
Despite a 55-kilometre (34-mile) deconfliction zone around the base, Al-Tanf is seen as a potential flashpoint between US and Iranian or Tehran-backed forces.
The presence of a US base in the arid border region has been a source of tension and its dismantling is often cited as a key demand by Damascus and its allies.
Beyond the battle against IS jihadists in their nearby desert hideouts, analysts say Washington sees the base as disrupting Iranian efforts to open a east-west land corridor from Tehran to Lebanon.