Video footage posted online Wednesday shows nine bodies of prisoners, blindfolded and hands bound, in pools of blood allegedly executed by Al-Qaeda-linked militants in northern Syria.
The footage, posted by Shahba Press, a network of activists in the region, shows the nine men were all shot in the head.
While recording the images, the cameraman is heard to say: "This was done by ISIL. Are the victims apostates? No, they're Muslims".
He was referring to the Islamic State of Iraq and the Levant, whose Aleppo headquarters was overrun by opposition militants earlier Wednesday as clashes between opposition forces raged into a sixth day.
One Shahba Press activist told AFP "dozens of prisoners were freed but dozens of others were executed on Monday or Tuesday by ISIL during the assault on their HQ" at a hospital in the Qadi Askar neighbourhood of Aleppo.
The jihadists' loss in Syria's second city came a day after ISIL's spokesman threatened to "crush" rebels who have attacked the group in multiple provinces.
Opposition Observatory chief "Rami Abdel Rahman" told AFP the killings in the headquarters were a war crime against humanity.