More than 650,000 people have been displaced or severely affected by fighting in two Sudanese states, the United Nations said on Wednesday, expressing hope that a newly signed aid deal will lead to access after months of restrictions.

The UN has expressed concern for months about a worsening humanitarian situation in the South Kordofan and Blue Nile warzone, but Khartoum cited security concerns in tightly restricting the operations of foreign relief agencies.

On Sunday, however, Sudan signed a memorandum of understanding with the UN, African Union and Arab League to allow for humanitarian access throughout the warzone -- including in rebel-held areas.

"It has taken 14 months to get to this point," Mark Cutts, the UN's acting resident and humanitarian coordinator, said after a mid-year briefing to aid agencies, diplomats and others.

"We greatly welcome the government's acceptance of this initiative and signing of a memorandum of understanding," he said, expecting "major progress" on reaching needy civilians over coming days and weeks.

A similar memorandum has been reached with rebels of the Sudan People's Liberation Movement-North (SPLM-N), who have been battling government forces since June last year in South Kordofan, and since September in Blue Nile.

Initial estimates are that 520,000 people have been displaced or severely affected in South Kordofan and another 145,000 in Blue Nile, Cutts said.