The top U.N. humanitarian official has visited Sudan to discuss a growing crisis in two southern states where Khartoum has banned foreign aid groups.
Valerie Amos says she has reports that food insecurity and malnutrition have reached "alarming" levels in parts of Southern Kordofan and Blue Nile states.
Fighting between Sudan's army and rebels in the two states broke out last year, a few weeks before South Sudan formally split from the north on July 9. The rebel groups were aligned with the south during Sudan's long civil war.
After meeting Wednesday with Sudanese officials in Khartoum, Amos said she urged Sudan to lift the ban on foreign aid groups and allow them access to the two states.
She said Sudanese officials agreed to provide the United Nations with humanitarian information, but did not grant access.
Amos said refugees who have fled to South Sudan and Ethiopia report the humanitarian situation is especially bad in areas held by the rebel SPLM-North group.
Fighting in the two states has raised concerns Sudan and South Sudan are on the brink of war.
The two countries have not resolved disputes over borders and oil revenue-sharing. They have also accused each other of supporting rebels in the other's territory.
Some information for this report was provided by AFP and Reuters.