Tensions have flared in the oil-producing state after South Sudan seceded last month, taking its oil fields with it. The Security Council said in July it was gravely concerned about violence in the volatile border territory.

UN officials have complained they are being granted only limited or no access to the area, which contains large populations that sided with South Sudan during the 20-year civil war.

“The complaint accuses South Sudan of causing instability, disrupting peace and offering support to rebel groups in the South Kordofan state,” a spokesman from Sudan’s Foreign Ministry told Reuters.

The Sudanese Foreign Ministry statement accuses South Sudan of providing military support to tribes in the area.

Sudan’s foreign minister, Ali Ahmed Karti, said in a copy of the complaint carried by the Sudanese news agency SUNA that Sudan was committed to stability and peace, but that the South Sudanese government was “hostile” toward its neighbor.

“We were the first to acknowledge an independent South Sudan and extended a hand of cooperation,” Kerti said, accusing Salva Kiir, president of the new South Sudan, of claiming South Kordofan and the Blue Nile province, both north of the new border.

“The newly-born state adopted a hostile attitude toward the state that gave it the right to self-determination, and that was evident when it ... hosted rebel groups from Darfur, offering them camps, shelter, training and equipping with arms to be used against the Sudanese republic,” he said.

Activists have accused Khartoum of launching airstrikes and attacks in Southern Kordofan, targeting the state’s ethnic Nuba group, in a bid to stamp out opposition and assert its authority after South Sudan’s independence.

Sudan’s government has dismissed the accusations and accused local armed groups, many of which fought alongside the south during decades of civil war with the north, of launching a rebellion to try to control the territory.

The United Nations said on June 22 that 73,000 people had fled Southern Kordofan after more than two weeks of fighting although some later returned to their homes.