Airstrikes hit positions of pro-Iranian forces and allied militias in eastern Syria overnight, killing 18 fighters, a war monitor said Monday.
It was not clear who carried out the raids in the region of Albu Kamal near the border with Iraq, according to the Syrian Observatory for Human Rights.
The Britain-based Observatory, which has a vast network of contacts across Syria, said: "18 fighters were killed, but their nationalities have not yet been determined".
Albu Kamal lies in Deir Ezzor province which covers much of Syria's remote eastern desert, where the ISIS group'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.
An American official said at the time that Israel was responsible, but the Jewish state declined to comment.
Israel has carried out hundreds of strikes in Syria targeting what it says are positions of Lebanese Shiite movement Hezbollah and Iranian forces, which it has vowed to prevent gaining a foothold on Syrian territory.
Iran, its allied militias and Russia have backed Syrian President Bashar Al-Assad in the country's devastating eight-year civil war.
Pro-regime forces in Deir Ezzor are operating with the backing of various foreign armed groups including Iraqis and Iranians.
The Kurdish-dominated Syrian Democratic Forces control a swathe of territory further north in the province, which it seized from ISIS in a months-long campaign backed by US-led coalition forces.
The Syrian conflict, which broke out in 2011 with the bloody repression of anti-regime demonstrations, has become a complex war, dragging in regional and international powers and leaving more than 370,000 people dead.