International rights groups on Tuesday accused Sudan's government of killing at least 26 people in indiscriminate aerial bombardments of a contested region in the country's main oil-producing state.
Airstrikes in the Nuba Mountain areas of South Kordofan state also wounded 45 people, Human Rights Watch and Amnesty International said in the statement.
The rights groups said that Sudanese government restrictions have prevented aid groups from delivering food and other assistance to more than 150,000 people displaced by the violence in the contested region.
"Indiscriminate attacks in civilian areas and restrictions on humanitarian aid could amount to war crimes and crimes against humanity," said Donatella Rovera, Amnesty International's senior crisis response adviser.
South Kordofan lies just across the border from South Sudan, which gained independence from the north on July 9, and has been the site of clashes between government troops from Sudan's Arab north and black tribesmen aligned with the south's Sudan People's Liberation Movement.
Many inhabitants of South Kordofan fought for the south during the country's two decades-plus civil war against the north and are ethnically linked to the south.
"The relentless bombing campaign is killing and maiming civilian men, women, and children, displacing tens of thousands, putting them in desperate need of aid, and preventing entire communities from planting crops and feeding their children," said Daniel Bekele, Africa director at Human Rights Watch.
Researchers from Amnesty International and Human Rights Watch investigated 13 air strikes in the Kauda, Delami, and Kurchi areas of South Kordofan during a week in late August, the statement said. The groups said they witnessed government planes circling over civilian areas and dropping bombs, forcing civilians to seek shelter in mountains and caves.
Sudan's government on Tuesday said the mountains are home to rebels armed by South Sudan and urged the U.N. to take action against the South Sudan for violating a 2005 peace deal that ended decades of conflict.
Ali Karti, Sudan's Minister of Foreign Affairs, complained in an official message to the U.N. Security Council of what he said were repeated recent violations of the peace agreement in the regions of South Kordofan, Abyei and the Blue Nile.
The statement also accused South Sudan of training and giving safe haven to rebels in Sudan's Darfur region, and of sending troops into the contested area of Abyei in May. The latter event spurred an international outcry and prompted Ethiopia to send in peacekeepers to keep the two sides apart.
The Foreign Ministry statement said the violations were "committed by the government of the Republic of South Sudan."
A U.S. monitoring group said last week that satellite imagery found two more mass graves in South Kordofan, bringing the total number of graves sited there to eight.
The U.S. group has not made any estimates of the number of bodies it believes have been buried in the graves, saying that onsite research would need to be carried out.
Associated Press Writer Mohamed Osman contributed to this report from Khartoum, Sudan.