At least 12 pro-Iranian fighters were killed in an unidentified airstrike in eastern Syria on Saturday evening.
The London-based Syrian Observatory for Human Rights said eight airstrikes had been carried out targeting Iranian forces in the eastern Deir ez-Zur area.
The Observatory for Human Rights did not say who carried out the attack, but its head, Rami Abdel Rahman, said Israel had probably carried out the attack.
Earlier, Israel carried out hundreds of airstrikes targeting Iranian-backed forces, especially in the Deir ez-Zur area.
Israel has rarely claimed responsibility for the attacks.