Leaders from South Sudan and Sudan met recently in Ethiopia to discuss issues left from a peace agreement signed in 2005 with Washington's help. That deal paved the way to South Sudan's independence in 2011, though ethnic tensions, oil disputes and border conflicts have threatened the peace.
U.S. Ambassador to the United Nations Susan Rice said the United States was concerned about reports of fighting in the area.
"We remain deeply concerned by the continued deterioration of the humanitarian situation in Southern Kordofan and Blue Nile," she said in a statement from U.N. headquarters. "We're also deeply concerned about the ongoing aerial bombardments by the Sudanese armed forces, including in civilian areas."
Witnesses in the region told the independent Sudan Tribune that tribal clashes along the border left around 80 people dead in South Kordofan state on Sudan's side of the border. The same source said security forces were outnumbered, noting fighting lasted for around 10 hours earlier this week.