KHARTOUM - There has been fresh fighting in Sudan's Blue Nile state, rebel and government forces said on Thursday, after the United States warned of a possible "horrific" famine affecting civilians in the area.

The rebels said they shot down a helicopter gunship sent in to rescue a convoy caught in a forest ambush that killed 26 members of the Sudan Armed Forces.

It happened on Wednesday near Boot village, about 45 kilometres (28 miles) west of the state capital Ed Damazin, said Arnu Ngutulu Lodi of the Sudan People's Liberation Movement-North (SPLM-N).

He said the rebels suffered no casualties, but Sudan's army spokesman, Sawarmi Khaled Saad, said his forces killed seven insurgents while six from the government side were wounded in the action near the South Sudan border between Monday and Wednesday.

"The fighting happened because SAF was trying to clean Al Roum area and defeat the rebels from this area," he said. "We didn't lose any helicopter gunship."

Fighting in Blue Nile erupted in September between the government and ethnic rebels, once allied to insurgents who now rule in South Sudan which became independent from Khartoum last July after decades of civil war.

A similar conflict began earlier last year in South Kordofan, a Sudanese oil-producing state.

The United Nations estimates that more than 500,000 people have now been displaced or severely affected by fighting in Blue Nile and South Kordofan, with many fleeing to neighbouring Ethiopia and South Sudan.

The government, citing security concerns, continues to bar UN and foreign aid workers from the war zone.

The area is at risk of famine without substantial aid deliveries by March, the American ambassador to the United Nations, Susan Rice, said on Tuesday.

Princeton Lyman, the US special envoy to Sudan, on Wednesday called on South Africa to pressure Khartoum to let international aid groups into Blue Nile and South Kordofan.

"There is time to prevent a horrific tragedy," Lyman said.

Sudan's UN ambassador on Tuesday accused international aid workers of using UN flights to carry arms and ammunition for the rebels -- a claim for which the UN's top humanitarian official said there was "no evidence."

Valerie Amos said the UN is negotiating with the African Union and Arab League about providing observers in South Kordofan and Blue Nile states, "to perhaps give the government of Sudan the confidence that they are looking for."

The UN backed statements by the United States that there could be a famine in the two states unless urgent aid is allowed in.