An ISIS guard has revealed he quit the terror group after witnessing a 15-year-old sex slave raped so horrifically that she bled to death.

Giving his name as Nizar, the terrorist has lifted the lid on life inside Islamic State's detention centres.

He recalled a teenager who died from internal bleeding after she was raped by a Sudanese jihadi in her cell.

The women captured by ISIS and given away as sex slaves to the group's terrorists

Three women cover their faces after being captured by Islamic State militants in Syria

Nizar was told not to speak of the incident, and when he questioned it, he told God Reports a colleague said: 'Captives are captives - do as you please with them.'

After ISIS captured his city in Syria, he thought it would bring positivity and became a guard at the religious police's centre.

'It really felt like Islam,' he said.

'We started going to the mosque. A sheikh would give lessons in the Quran.'

The groups twisted notion of Islam quickly became apparent as he held up to 100 people men a day who were facing charges ranging from smoking to having a wife who was not appropriately covered up.

'They were imprisoned, whipped and beaten,' he said.

'After two or three days they were released.'

Inside the detention centres in Syria where ISIS' religious police would dish out justice

More worrying were the 475 female captives, which included Yazidi and Iraqi women, who seemingly were often locked up just for being the wives of opposition soldiers or potential criminals.

Nizar said: 'When I walked down the corridor, I was shocked looking at all these women.'

He told God Reports how women would cry because they had been separated from their children, and any new arrivals would prompt cheers from ISIS fighters.

The buoyant terrorists would be brought in 10 at a time to pick their sex slave for the next three days.

After which, they would either be abused or married off to strangers and the alternative was often execution.

An Islamic State militant brandishes a weapon at some clearly petrified women and children

Nizar began to have nightmares akin to PTSD about his work, and decided he needed to abscond.

'I felt tired and uncomfortable,' he said.

'I lost my appetite. I thought of them as my sisters.'

He and his family ran away to Turkey, and has advised men to steer clear of Syria.

'Once you get in the sand, it will turn into quicksand – as soon as you're in it, you drown,' he said.