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.

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

When ISIS swept through the Yazidi homeland, they rounded up the residents from the nearby areas on the road into Sinjar. The residents, including the boys who refused to be conscripted by the terrorist group, were killed. Witnesses told CNN that more than 130 bodies were in the mass graves there.

ISIS has also dumped bodies into mass graves in Syria. A mass grave of about 40 women and children buried in the city of Palmyra was discovered in April. The Syrian news agency reported that the victims bore signs of beheading and torture.