Warplanes have struck positions of Iran-backed militias near Syria's border with Iraq, activists say.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said at least 18 Iranian and pro-Iranian fighters were killed.
It was not clear who carried out the overnight strikes in and around the town of Albu Kamal.
But Israel has carried out hundreds of attacks on Iranian-linked targets in Syria during the country's civil war.
It has sought to thwart what it calls Iran's "military entrenchment" in Syria and shipments of Iranian weapons to militant groups such as Hezbollah in Lebanon.