The United Nations says there is “strong evidence” 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.

In a separate statement Friday, the United States also said it was “deeply concerned” about “alarming and credible” reports of violence by Sudanese forces and its allies. U.S. Ambassador to the U.N. Susan Rice said the reports include “acts of extreme cruelty” against civilians.

Both statements said if the allegations are proven true, the acts could amount to crimes against humanity.

U.N. 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.

U.S. Ambassador Rice said the U.S. would support an investigation by the U.N. human rights office. She said the U.S. also called for Sudan's government to accept a “robust U.N. presence” in the region to assist with implementing any future political and security agreements.

Earlier this month, the U.N. Security Council voted to withdraw the U.N. peacekeeping force sent to Sudan to monitor the 2005 peace agreement that ended the Sudanese civil war, after unsuccessfully appealing to the Khartoum government to allow the peacekeepers to remain to deal with unresolved issues from that agreement.

Both the U.S. and British governments said they voted in favor of withdrawing the peacekeepers with regret.