The United States is urging Sudan to allow independent investigators into its Southern Kordofan state after the United Nations reported alleged war crimes there.

The U.S. ambassador to the United Nations, Susan Rice, said Tuesday the situation is becoming “increasingly dire” in Southern Kordofan, where forces allied to what were then northern and southern Sudan clashed in June.

She said the United Sates is deeply disturbed by reports of extrajudicial killings, attacks on civilians, mass graves, and other violations of humanitarian law. Rice said the administration of U.S. President Barack Obama strongly backs the United Nations' call for an independent inquiry in Southern Kordofan.

A report by the U.N. human rights office Monday said beginning June 7 in the town of Kadugli, northern forces shelled civilian areas, executed pro-southern residents, and looted and destroyed civilian homes.

The fighting preceded South Sudan's split from the north in July, six years after a long civil war. The Khartoum government had ordered former southern fighters in the state to disarm or move south.

The U.N. report blames most of the abuses in Southern Kordofan on the north's army, police, and allied militia.

Sudanese President Omar al-Bashir is wanted by the International Criminal Court on charges of war crimes and genocide in Sudan's western Darfur region.