(CNN)More than 200 mass graves containing the remains of thousands of victims have been found in areas formerly controlled by ISIS, a United Nations report revealed Tuesday.
The grave sites, which may contain up to 12,000 bodies, were found in the northern and western Iraqi provinces of Nineveh, Kirkuk, Salah al-Din and Anbar.
The smallest grave site, found in west Mosul, contained eight corpses, the report says, and the largest -- which is believed to be the Khasfa sinkhole south of Mosul -- may contain up to 4,000 bodies.
ISIS -- the militant group that is also known as ISIL -- seized large areas of Iraq between June 2014 and December 2017 and members declared them as part of a so-called caliphate.
The report notes that the group led "a campaign of widespread violence and systematic violations of international human rights and humanitarian law."
Forensic material could prove war crimes
The graves, the report says, could contain critical forensic material that will not only be able to help identify victims, but also "build an understanding around the scale of abuses and violations that occurred" and determine if acts amount to war crimes, crimes against humanity or genocide.
UNAMI and OHCHR said ISIS' victims include women, children, elderly people and those with disabilities, members of Iraq's armed forces, police and some foreign workers.
The director of human rights at UNAMI, Suki Nagra, told CNN that the key message was that the excavations and exhumations should be appropriately protected and preserved.
"The mass graves should be treated as crime scenes and that any evidence that's extracted from them should be used in criminal prosecutions in the future in line with international standards.
"For us, the biggest issue is that the truth comes out of what actually happened -- for the victims -- and that the evidence from the results of the exhumations from these mass graves is actually used for criminal prosecutions," Nagra said.
"ISIL's horrific crimes in Iraq have left the headlines but the trauma of the victims' families endures, with thousands of women, men and children still unaccounted for," UN High Commissioner for Human Rights Michelle Bachelet said.
"These graves contain the remains of those mercilessly killed for not conforming to ISIL's twisted ideology and rule, including ethnic and religious minorities."
The report also highlighted the difficulties Iraqi authorities have faced when trying to exhume remains.
"In several areas where ISIL remains active ... their continued presence may inhibit the ability of investigators to access sites and to carry out their work unhindered," the report says, adding that additional security may be required to carry out long-term investigations.
So far, it says, the remains of 1,258 people have been exhumed from the sites by the Mass Graves Directorate.
The report has called on the international community to provide assistance with the exhumations, identification of victims and the return of remains to families.
"The mass grave sites documented in our report are a testament to harrowing human loss, profound suffering and shocking cruelty," said Ján Kubiš, special representative for Iraq of the UN Secretary-General.