The Syrian Observatory for Human Rights said the bodies of the 34 were discovered "executed" after the clashes in Jabal Zawiya, adding that all the dead were non-Syrian fighters.
The group said the 34, from ISIL and a group called Jund al-Aqsa, were killed by non-jihadist rebels over the past few days in the northwestern province.
The deaths came amid fighting on several fronts pitting several coalitions of rebel fighters against jihadists from ISIL.
Syria's rebels, many of them civilians who took up arms against the regime, initially welcomed the arrival of battle-hardened jihadists to the fight.
But tensions have mounted, with activists accusing ISIL of imposing a reign of terror, and rebels saying the group has focused on accumulating territory and fighting other rebels rather than targeting the regime.