Beirut - The Syrian Observatory for Human Rights said on Friday that an al-Qaeda splinter group in Syria executed at least 21 people, including fighters from rival rebel groups and their relatives.
The incident could worsen infighting among enemies of President Bashar al-Assad.
The Islamic State of Iraq and the Levant, which has attracted many foreign militants into its ranks, is a small but powerful fighting force in Syria that emerged from the Sunni Islamist insurgency in neighbouring Iraq. In Syria, it has alienated many civilians and opposition activists by imposing harsh rulings against dissent in areas it controls.
The observatory said ISIL militants had executed 17 men in Haritan, just northwest of Aleppo, one of the most contested areas in an almost three-year-old civil war between insurgents and Assad's forces.
The bodies of the men, who included rebels who were detained by ISIL after laying down their arms, were thrown into a well.
ISIL militants also beheaded at least four fighters from rival rebel factions after capturing them during fighting in Azaz, north of Aleppo near the border with Turkey, said the Observatory, a London-based monitoring group.