Over 50 mass graves have been found so far in different areas of Iraq formerly controlled by the Islamic State group, also known as ISIS, BBC reported Friday, citing a United Nations envoy.

Jan Kubis, U.N. special representative in its assistance mission for Iraq, told the U.N. Security Council that the mass graves were “evidence of the heinous crimes” ISIS committed. He added that the international community should “take steps to ensure the accountability” of ISIS militants, BBC reported.

“I condemn in the strongest possible terms the continued killings, kidnapping, rape and torture of Iraqis by ISIL [ISIS], which may constitute crimes against humanity, war crimes and even genocide,” Kubis reportedly said.

Human remains have been uncovered in mass graves near Sinjar in northern Iraq, near Anbar in western Iraq and in Tikrit in northern Iraq. In the city of Ramadi, three graves containing remains of 40 people were found in a football field on April 19, according to Kubis. The Iraqi forces took back Ramadi — held by ISIS from May 2015 — last December. The victims included tribesmen, Iraqi soldiers, women and people from the minority Yazidi sect.

The U.N. envoy said that despite “notable and consistent progress” against the Islamic terror group, it was still “a formidable and determined enemy that constantly adjusts its tactics and attack patterns.” He called for addressing the “the root causes of violent extremism,” saying that ISIS would not be defeated only by military.

More than 10 million people in Iraq need international aid and the country’s  humanitarian crisis “remains one of the world's worst,” Kubis said, adding that a quarter of the $861 million requested for aid to Iraqis in 2016 had been delivered till now.

Mass graves have also been found in several areas of Syria previously controlled by ISIS.