At least 12 pro-Iranian fighters died in strikes by unidentified aircraft on eastern Syria late Saturday evening, a war monitor said.
"Eight air strikes before midnight on Saturday night targeted a base of pro-Iranian forces in rural eastern Deir Ezzor (province), killing 12 Iraqi and Afghan fighters and destroying equipment and ammunition," the Syrian Observatory for Human Rights said.
The Observatory did not identify the aircraft responsible, but its head Rami Abdul Rahman told AFP that Israel was likely responsible.
Israel has carried out hundreds of strikes targeting regime and Iranian-backed forces, notably in Deir Ezzor.
Iranian and Iraqi armed groups backing the regime of Bashar Al-Assad have deployed across swathes of Deir Ezzor, a large desert province bordering on Iraq.
The Observatory said the latest strikes came after Afghan forces brought in reinforcements from near the Iraqi border to a large Iranian base near the town of Al-Mayadin on the Euphrates river.
Two waves of similar strikes in May killed 12 pro-Iranian fighters, according to the Observatory.
Syria's complex, almost decade-long war has killed over 380,000 people, devastated the country's infrastructure and forced millions of people to flee their homes.