Iraq will use DNA testing to identify the remains of 141 bodies found in mass graves believed to contain the Yazidi victims of ISIS group massacres, the head of the country's forensics administration said on Sunday.

Zaid al-Yousef said Yazidi survivors helped to locate the 12 graves in the Sinjar region in north Iraq, reported the Associated Press.

Iraq is working to exhume remains from mass graves for forensic evidence of the ISIS group's crimes when it ruled over parts of the country's north from 2014 to 2017. It is supported by a special UN investigations team.

ISIS followers said the Yazidis were "apostates" and killed hundreds of men while enslaving thousands of women and children. The UN called it a genocide.

Yazidi groups say the process of exhuming graves and identifying victims is moving too slowly.

"We feel there is some neglect of the issue," said Ali Khedhir, the head of the Yazidi spiritual council office in the village of Lalesh, the spiritual home of the faith.

Tens of thousands of Yazidis are still living in camps outside Sinjar. Widespread destruction of their homes and persistent insecurity have dissuaded them from returning to their villages.

In March, Iraq and the UN began exhuming graves in the village of Kocho, where ISIS extremists are believed to have massacred hundreds of Yazidis in August 2014.

Another 3,000 Yazidis are still missing after they were abducted from Sinjar by ISIS. The survivors are believed to be living among ISIS families held in detention by Syrian Kurdish forces in neighboring Syria.

Al-Yousef, the director of Iraq's Department of Forensic Medicine, told The AP on Sunday it will take until August to identify the 141 bodies transferred to Baghdad from Sinjar for DNA testing.

The investigation is being hampered by a lack of survivors and hesitation among relatives of the dead to come forward with DNA samples, said al-Yousef.

Investigators require DNA samples from three relatives to identify each victim. The UN is searching for relatives of the deceased in other countries and seeking to obtain DNA samples to assist the investigation, he added.

"The Yazidi investigation is the most difficult we have ever come across," said al-Yousef.