It is one of the most chilling images to emerge from rebel-held Syria. In front of an audience of schoolchildren, the masked executioner pushes his trembling victim to the ground and slices his blade across the man's throat.
A series of photographs, too graphic to publish, show the slaughter of four prisoners held by jihadist groups at different public execution sites outside Aleppo. Little is known about the victims; only that they were accused of being Shabiha - pro-government paramilitaries.
Grim footage depicting a litany of crimes - murders, torture and mass killings - now emerge on YouTube on an almost daily basis from the Syrian civil war. But their authenticity is often impossible to verify. But these pictures, first published in the French magazine Paris Match, were taken by an independent photojournalist whose name, for his own safety, has not been publicly revealed.
His photographs graphically illustrate the callous barbarism of al-Qaeda-linked rebels that have now grown dominant in large parts of northern Syria. In many districts of Aleppo, historically a place of learning and trade, hardline Sharia or religious Islamic courts dictate the law of the land. Al-Qaeda's interpretation of Sharia has dragged parts of Syria back to a medieval age, where sentences for crimes and misdemeanours range from public floggings to execution by beheading.
Though still a minority among the rebels fighting the regime of Bashar al-Assad, the Islamic State of Iraq and Syria (ISIS) has grown in prominence in recent months in Syria. Better armed, and willing to use extreme fighting tactics, including suicide bombings, the group has been at the forefront of most of the major attacks against regime installations in the north of Syria. With Jabhat al-Nusra, a rival al-Qaeda linked group, ISIS has also taken control of much of the region's income-generating resources, such as oil, gas and grain.
Yesterday, Ayman al-Zawahri, the leader of al-Qaeda, issued an audio tape in which he warned his groups in Syria against collaborating with more moderate rebels. In the past few months, ISIS fighters have been asserting their dominance, assassinating prominent commanders of the more moderate rebel group, the Free Syrian Army.