At least eight pro-regime fighters were killed in an air strike near a US base in southeastern Syria, said the Syrian Observatory for Human Rights on Monday.
"A convoy of Iranian forces and allied militia was hit by air strikes as it drove near Al-Tanf base," Observatory head Rami Abdel Rahman said.
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 the Observatory.
Abdel Rahman 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".
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 the regime and its allies.
Beyond the battle against ISIS in their nearby desert hideouts, analysts say Washington sees the base as disrupting Iranian efforts to open an east-west land corridor from Tehran to Lebanon.
