(Updates with larger number of executions)
BEIRUT, Feb 14 (Reuters) - An al Qaeda splinter group in Syria executed at least 21 people including fighters from rival rebel groups and their relatives, a monitoring group said on Friday, an incident that 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 Syrian Observatory for Human Rights 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.
Several Islamist and more secular rebel factions joined forces in January for an offensive to try to push their ISIL former allies out of rebel-held regions in northern and eastern Syria.
Hundreds of people have been killed in the fighting, undermining the unity of the opposition to Assad. (Writing by Tom Perry; Editing by Mark Heinrich and Peter Graff)