It turns out that the Islamic State (ISIS) is not only a group composed of ruthless killers and savages but also of monstrous cannibals.

In a shocking revelation, an ISIS handbook has been discovered justifying and teaching cannibalism among the group's members, British media reported.

The document was found in one of the terrorist group's training centres and presented to the media by the Quilliam Foundation, a counter-extremist think tank founded by three former members of the Hizb ut-Tahrir radical Islamist group.

The handbook tells aspiring militants that Islam approves of killing captive non-Muslims and feasting on their remains should food become scarce, according to The Daily Mail.

The guidebook even provides details on the parts of the body that can be eaten and how these body parts should be prepared and cooked.

"We have found the curriculum that Islamic State use to indoctrinate fighters and the indigenous population in areas under their control," Quilliam CEO Haras Rafiq told The Daily Record. "There are all sorts of things in there, but one of the most horrific is a section on cannibalism which they are teaching in classrooms."

Rafiq said ISIS militants are being told that if they can't find food during a time of jihad, "it is OK to kill another non-Muslim or a Muslim who doesn't follow their version of Islam."

Rafiq said as a Muslim he finds no justification for cannibalism, which he called "revolting."

"These views on cannibalism are not mainstream but they are gaining traction," Rafiq said.

This is not the first time that ISIS has been linked to cannibalism. There have been reports on ISIS militants cooking the remains of non-Muslim captives they butchered and feeding them to the victims' family members.

In October 2015, ISIS militants reportedly fed a detained and starving Yazidi mother the meat of her very own three-year old boy whom she had not seen since being kidnapped by the militants some months back.

After being locked up in an ISIS cell for two days without food, the woman begged to be fed. The militants obliged, giving her a plate of rice with meat. But after she finished eating, to her utter horror the militants told her that what she had just eaten was the meat of her own son.

A Kurdish mother also had a similar experience when she went to the ISIS headquarters in Mosul, Iraq in March 2015 to beg for the release of her detained son, who was a Kurdish soldier.

The ISIS militants pretended to be gracious hosts, telling her to sit down and even served her a meal of cooked meat, soup, and rice.

After she finished the meal, she demanded to see her son. The militants reportedly burst into laughter, saying they had murdered her son and chopped him into pieces. "You've just eaten him," they told her.