Islamic State militants have released sickening images of the public crucifixion and murder of a teenage boy.

The barbaric punishment took place over three days in the central square of the extremists' de facto capital Raqqa.

The militant group accused the teenager of taking photographs of the terror group's headquarters in Syria. It's claimed the boy was caught receiving 500 Turkish lira for every photograph he took of the Islamic State military base.

The graphic images show the battered and bloodstained body of the unnamed boy strapped to a cross, with a handwritten placard hanging around his neck, accusing him of apostasy.

A handful of individuals who oppose ISIS have attempted to document the violence the terrorist group has inflicted in Raqqa.

The image has been circulated on Twitter both by anti-ISIS campaigners and by ISIS supporters.

Charlie Winter, programs officer at counter-extremism think tank the UK Quilliam Foundation, said crucifixion is a prescribed punishment meted out by Isis for specific crimes.

"Crucifixion has been used many times before – it's an age-old punishment dealt out to people who have committed treason," he said.

The militant group have embarked on a reign of terror since they seized control of the city earlier in the year.

The city has been bathed in blood, with severed heads regularly impaled on spikes on a small patch of grass in the square, and wooden and metal crosses readied for executions.