The United Nations called Monday for a thorough probe into alleged war crimes and crimes against humanity committed in Sudan's South Kordofan region between June 5-30, shortly before the nation formally separated into two countries.

According to a 12-page report released by the U.N. human rights office in Geneva, Sudanese security forces allegedly carried out indiscriminate aerial bombardments that killed civilians, and executed prisoners accused of belonging to the south's Sudan Peoples' Liberation Movement before burying them in mass graves.

The report also cites claims that both the Sudanese army and members of the SPLM laid anti-personnel mines in Kadugli, the capital of South Kordofan.

Many inhabitants of South Kordofan fought for the south during the country's decades-long civil war against the north and are ethnically linked to the south. Clashes between government troops from Sudan's Arab north and forces aligned with the south have continued after South Sudan's independence on July 9, straining relations between the two countries.

"This is a preliminary report produced under very challenging circumstances and with very limited access to affected areas," the U.N. High Commissioner for Human Rights Navi Pillay said in a statement. "However what it suggests has been happening in Southern Kordofan is so serious that it is essential there is an independent, thorough and objective inquiry with the aim of holding perpetrators to account."

In one instance, the report cites a former detainee at a Sudan Armed Forces facility in Kadugli who says he saw over 150 bodies scattered on the grounds of the military compound, some with bullet wounds.

In another case, a U.N. contractor who was later identified as an SPLM member was pulled out of a vehicle by Sudanese soldiers and shot. The alleged incident took place near the U.N. compound at Kadugli, the report said.

The report also cites claims that Sudanese forces used chemical weapons against civilians and dropped bombs in civilian areas including close to a school.

"The violations described in this report (...) if substantiated, may constitute war crimes and crimes against humanity," the U.N. said.

Last week, Russia and China blocked U.S. attempts to get the U.N. Security Council to condemn the Sudanese government bombing and other military activities in South Kordofan.