The Syrian Observatory for Human Rights said government forces had allowed the Islamic State fighters to leave a besieged pocket of territory at the intersection of Aleppo, Idlib and Hama provinces, and to go to southern Idlib.
A military news outlet run by the Lebanese group Hezbollah, which is fighting on the Syrian government side, reported gains for the army and its allies against Islamic State fighters in that pocket, but made no mention of them being allowed to leave.
The Syrian military could not be reached for comment.
"The regime started the operation against this pocket seven days, suddenly they were able to take 80 villages and towns after giving them a corridor," Observatory Director Rami Abdulrahman said.
Hasan Haj Ali, commander of the Free Idlib Army rebel group, confirmed the report and said his fighters were taking part in clashes with some 200 Islamic State fighters who had arrived in southern Idlib early on Friday.
"This morning at dawn we were surprised by the joint treachery by the regime and Daesh," he told Reuters. Clashes were underway in the village of Lweibdeh, he said. "They have six armoured vehicles with them."Idlib is the largest chunk of Syrian territory held by groups opposed to President Bashar al-Assad. Islamist factions including al Qaeda's former affiliate in the Syrian war dominate the province.