United Nations: UN Chief Ban Ki-moon has called for an immediate end to clashes in the Sudanese state of South Kordofan that are threatening to cast a pall of gloom over southern Sudan's imminent independence.

Heavy fighting between the northern Sudanese Armed Forces and allied militiamen against fighters aligned to a group of southern former rebels, the Sudan People's Liberation Army, has raged across the state since June 5.

The international Catholic charity Caritas says more than 60,000 people have fled their homes, while a UN report said the Sudan Red Crescent Society had collected 25 dead bodies in the state capital Kadugli.

"The secretary-general is deeply concerned about the deterioration of the security situation and the escalation of fighting in Southern Kordofan, which has caused the death of many civilians and the displacement of tens of thousands, and put United Nations staff directly at risk," Ban said in a statement on Wednesday.

The UN chief also called for the UN peacekeeping mission in Sudan (UNMIS) and humanitarian workers to be allowed into South Kordofan immediately so that vital assistance can be provided to the affected population.

The World Food Program said the lack of security was preventing it from providing emergency supplies to tens of thousands of people uprooted by the conflict, now thought to number more than 64,000.

The uptick in violence in Sudan, including fresh clashes on Wednesday in the second border state of Abyei, comes less than a month before southern Sudan is supposed to officially declare independence from the north on July 9.

Complex racial and religious rivalries were driving forces in Sudan's 1983-2005 civil wars in which more than 1.5 million people were killed.

Tension remain high between the Arab-dominated north and the non-Arab south, where inhabitants are either Christians or followers of traditional beliefs.