Islamic State militants have killed 35 Syrian pro-government forces in desert attacks in recent days, the Syrian Observatory for Human Rights says.
The UK-based monitoring group says the militants attacked in Homs and Deir al-Zour provinces.
IS media has spoken about the alleged attacks, but Syrian officials have not confirmed them.
It comes weeks after reports some IS militants had fled into the desert from Baghuz - their last stronghold.
The area was declared "freed" by the Kurdish-led Syrian Democratic Forces (SDF) on 23 March.
Although the declaration marked the last territorial victory over the group's caliphate, experts warn it does not mean the end of IS or its ideology.
Thousands of fighters and their families captured from Baghuz, including foreign nationals, remain in camps nearby.
The Syrian Observatory for Human Rights say IS militants have killed 27 government troops and allied militia in the desert in the east of Homs province since Thursday.
Another eight were killed in the province of Deir al-Zour on Thursday night, the monitor reports.
Observatory chief Rami Abdel Rahman described it as the "biggest attack and the highest death toll among regime forces since the caliphate was declared defeated".
At least six IS militants were also killed in the clashes, the monitor says.
The IS group's news outlet, Amaq, allege its militants were able to seize army weapons during the Homs clashes, including a number of armoured vehicles and machine guns.