The Islamic State of Iraq and the Levant (ISIL) executed "a rebel battalion leader, accusing him of insulting God and of apostasy," AFP quoted the opposition so-called Syrian Observatory for Human Rights as saying on Wednesday.
The battalion has been locked in combat against ISIL, which has pressed hard to crush other terrorist groups in areas across eastern and northern Syria.
ISIL executed the rebel leader after holding him for three days, the Observatory said.
Syrian activists have repeatedly denounced abuses committed by militants, who have taken advantage of Syria's war to establish a presence in the country.
BA/BA