Air strikes killed at least 18 Iranians and pro-Iranian fighters in eastern Syria.
The Syrian Observatory for Human Rights, a pro-opposition monitor of the Syrian war that reports from Britain, said the strikes started on Sunday night and continued on Monday. They caused extensive damage to an arms depot in Al Bukamal in eastern Syria.
Two local activist collectives in eastern Syria – the Euphrates Post and the Sound and Pictures – also reported the strikes.
The strikes came before Israel said that rockets were fired towards its territory by an Iran-backed force in Syria. None of the rockets reached Israeli territory.
Al Bukamal lies in Deir Ezzor province, which covers much of Syria's remote eastern desert, where ISIS's so-called "caliphate" made its last stand this year.
Control of the area is split between US-backed Kurdish fighters and groups aligned with the Damascus regime, which is supported by Iran and Russia.
In June 2018, strikes near the Iraqi border killed 55 pro-regime forces, mostly Syrians and Iraqis, the Observatory said.