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 airstrikes against pro-regime fighters.
The coalition is backing Kurdish-led fighters on the eastern shores of the Euphrates.