In August of 2014, Islamic State militants began systematically killing and enslaving Yazidi people in northern Iraq as part of their campaign to eliminate the religious minority deemed by the jihadists as unclean "devil worshippers." Since then, survivors and human rights advocates have documented more than 35 mass graves across the region, according to a new report exclusive to VICE News.

Yazda, a Yazidi rights group based in the Kurdistan region of northern Iraq, began discovering and cataloguing the sites as part of its genocide documentation last year after Sinjar was retaken by Kurdish forces. The mass graves provide important evidence of the atrocities committed by IS, however, since they are currently unprotected by the government, they are vulnerable to tampering and theft, the report's author warns.

"Reporting on these mass graves and preserving them is not just about feeling sad for the Yazidis who have survived the attacks by IS, it's about understanding what happened so that the international community can be directed to rebuilding the Yazidi homeland," Matthew Barber, Yazda's executive director, told VICE News from Dohuk.

"And for that to be done, we need professional forensic specialists here to test these sites, collect all the evidence, and ensure that families can then begin to rebury those remains and begin to heal."

Skulls from a kill site near Qane. Photo provided to Yazda by an anonymous Yazidi fighter

The Yazidis, a religious minority from northern Iraq with an estimated population of 600,000 worldwide, espouse a complex system of beliefs and practices that contain elements of Christianity, Islam, and Zoroastrianism.

IS has kidnapped more than 2,000 Yazidi women and girls as sex slaves. More than 400,000 Yazidis have fled from their homes in Sinjar and Nineveh Plains. Many are currently living in government camps controlled by the Kurdish Regional Government (KRG).

Yazda members documented 19 of the graves in the report through survivor testimony and corroborating their stories by traveling directly to the sites. The remaining sites either remain under IS control or have been identified by survivors and Human Rights Watch. The report notes that this list is not exhaustive and that more sites are expected to be discovered with time.

There is no exact definition of how many bodies constitute a site as a mass grave, but generally, it's a site containing more than four. At most of the mass grave sites, IS soldiers have covered the dead bodies with soil, sometimes putting them into a pit. In a number of instances, bodies were left on top of the ground and were covered with dirt by bulldozers. The report also differentiates between the mass graves and 33 kill sites, which are locations where IS killed a small number of people, but did not bury them.

"What we know based on the graves, is that these massacres occurred predominantly for people who refused to convert. We know that women were enslaved and we know that Yazidi cultural sites such as holy places were destroyed," says Barber.

Barber explained that when it comes to burying their dead, Yazidis abide by a number of ancient customs including ensuring the head faces east, and the feet west, so that the face of the buried person will always be greeted by the morning sun.

Mass grave at Wadi al-Zleliya. Photo via Yazda

One mass grave site had 68 bodies, all of which were exhumed by the KRG under "unknown circumstances." Another site, a shallow pit discovered by Yazda team members last November, is said to be where 21 Yazidis were slaughtered, although the exact total is unknown. The site was found to have been tampered with as exposed bones and shell casings had been put into piles.

"Investigations into the crimes of genocide that IS waged against the Yazidis should be conducted now — on the part of the [International Criminal Court] and other relevant institutions — because the evidence is now available," the report concludes.

Even though Iraq is not a member of the International Criminal Court, Yazda and the Free Yezidi Foundation submitted another report to the court last September urging it to investigate the actions of IS against the Yazidis as genocide.