The strikes late Saturday targeted "positions of Iranian forces and allied militias" on the edge of the town of Albukamal, the Syrian Observatory for Human Rights said.

"Five non-Syrian fighters were killed," Observatory chief Rami Abdel Rahman told AFP, without being able to provide their nationalities.

Regime troops, Iranian forces and allied Iran-backed fighters, including from Iraq, are present in the area on the western banks of the Euphrates River, he said.

According to the Observatory, 10 Iraqi fighters were killed in September in air strikes of unknown origin in the same area.

At the start of that month, air raids killed 18 pro-Iran fighters, the monitor reported.

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 on what it says are positions of Lebanese Shiite movement Hezbollah and Iranian forces, which it has vowed to prevent gaining a foothold in Syrian territory.

But the US-led coalition that has been fighting the Islamic State group has in the past also admitted to carrying out air strikes against pro-regime fighters.

The coalition is backing Kurdish-led fighters on the eastern shores of the Euphrates.