The United Nations says there is “strong evidence” that atrocities are being carried out by Sudanese forces in the Southern Kordofan area.

In a statement Friday, U.N. human rights experts cited “credible reports” of ongoing mass killings, arbitrary detentions, kidnappings, aerial bombardment and attacks on churches.

The U.N. said if proven true, the acts could amount to war crimes and crimes against humanity.

Minority expert Gay McDougall said it appears people of Nuban descent in particular are being targeted. The Nuba people largely backed the south during a 21-year north-south civil war.

Southern Kordofan is controlled by Khartoum but many of its residents are sympathetic to South Sudan, which split from the rest of Sudan earlier this month.

Sudanese officials have characterized the fighting in Southern Kordofan as a rebellion.

The U.N. has called for an independent probe of the violence and urged Sudan to grant investigators unrestricted access to the region.