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.

A US-led coalition has been backing Kurdish-led forces fighting Islamic State group fighters on the eastern shores of the river.

The coalition has in the past admitted to carrying out air strikes against pro-regime fighters, but it denied any involvement in Saturday's air raids.

In a statement to AFP on Sunday it said the coalition "did not conduct any strikes in Syria (on) December 7, 2019".

Saturday's raids were the latest in a string of similar bombardments.

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

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

In June 2018, strikes near the Iraqi border killed 55 pro-regime forces, mostly Syrians and Iraqis, the monitor 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.