At least 60 regime troops and allied militiamen have been killed in two days of attacks by the ISIS group in various Syrian desert regions, said the Syrian Observatory for Human Rights on Saturday.
It said four senior regime officers were among the troops and militiamen killed in the desert east of Homs province over the past 48 hours.
Another eight regime forces and militiamen, including two officers, were killed in a separate attack in neighboring Deir Ezzor province on Thursday night.
The attack targeted a desert village south of the city of Mayadeen, upstream from the stretch of the Euphrates Valley, the Britain-based war monitor said.
On Saturday, extremists linked to Syria's former Al-Qaeda affiliate attacked loyalists outside the northwestern region of Idlib, killing 26 pro-regime fighters, it said.
Six ISIS fighters were killed in the clashes.
Although ISIS lost its last territorial enclave in Syria at Baghouz near the Iraqi border last month to US-backed Syrian Kurdish Democratic Forces (SDF), it still has fighters holding out in the remote central desert.
The Observatory said the attacks constituted ISIS’ biggest military operation since losing Baghouz and involved assaults in several parts of central Syria.
ISIS said in a message on its Amaq news outlet on Friday that it had killed 20 Syrian regime forces and injured others in the al-Sukhna region between Palmyra and Deir Ezzour in an ambush and clashes.
Amaq said that ISIS had launched its assault on Thursday evening after regime forces tried to track down its fighters.
Syrian state news outlets did not carry any reports of ISIS attacks or of deaths among pro-regime forces in clashes with the militant group.
The vast Syrian desert, known in Arabic as the Badia, stretches all the way from the capital Damascus and the cities of Homs and Hama to its north to the Euphrates Valley near the Iraq border.
Commanders of the US-led coalition, which provided air and artillery support for the SDF operation, have warned repeatedly that the extremists' loss of their last piece of territory did not mean their elimination as a fighting force.
Analysts have said that continuing search and destroy operations by the multiple alliances lined up against ISIS would be necessary to prevent them mounting a comeback from their desert hideouts.