(CNN) More than 50 mass graves have been discovered throughout Iraq as it regains cities and villages from ISIS, a U.N. special representative for the country said.

"As territory is retaken from the criminal and terrorist gangs of Daesh [also known as ISIS], evidence of the heinous crimes they have committed continues to be uncovered," Jan Kubiš told the U.N. Security Council on Friday.

Three mass graves with bodies of about 40 people were found in April, he said. They were buried in an area around a football field in central Ramadi, he said.

He said the violent acts by ISIS -- killing, kidnapping, rape and torture -- constitute crimes against humanity, war crimes and even genocide.

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There have been previous reports of mass graves perpetrated by ISIS.