An offshoot of the Jund al-Aqsa group killed more than 150 members of rebel factions in the village of Khan Sheikhoun in southern Idlib province, the US-based monitoring service reported.
Dozens of those murdered were members of a Free Syrian Army (FSA) faction, it said, citing a pro-al Qaeda social media outlet.
A bloody battle between Syrian Islamists resulted in the death of 150
Jund al-Aqsa is seen as ideologically close to Islamic State, al Qaeda's main jihadist rival.
Fighting between Jund al-Aqsa and Tahrir al-Sham has flared in the past week, in clashes that war monitors say have killed dozens.
Conflict in Syria Wed, November 16, 2016
Russia resumes strikes against Syria.
Play slideshow REUTERS 1 of 98
Still image shows Russian Bastion coastal missile launchers launching Oniks missiles at unknown location in Syria
Many of the murdered were member of the Free Syrian Army
Those clashes have added to the complexity of insurgent infighting in the west of the country.
Both Tahrir al-Sham and Jund al-Aqsa are also fighting against FSA rebel factions who have been foreign-backed.
Get Quotes on Home Insurance
Fighting between the different factions has flared up recently
Jihadist groups attacked the FSA for sending delegates to peace talks in Kazakhstan last month.
Many of those FSA groups are now fighting under the banner of the more moderate but powerful Islamist Ahrar al-Sham.
- Syria would have fallen into the grasps of TERRORISTS without Russia
- European Union supports ISIS, claims Syrian president
- Aunt of Alan Kurdi blasts West for doing 'NOTHING' to end war in Syria