Up to half a million starving refugees could flee violence in Sudan within months, sparking a massive food crisis in newly independent South Sudan, the UN's World Food Programme (WFP) has warned.

WFP's Deputy Executive Director Ramiro Lopes da Silva said relief agencies were preparing for the worst, with Sudan's government in Khartoum blocking emergency aid into border regions where it is battling rebel forces.

"The numbers being used for planning purposes in the context of a worst case scenario, we are speaking of somewhere between 300-500,000 people" set to flee to South Sudan, Da Silva told reporters.

"In a couple of months we are in what is typically the hunger season, both in Sudan and South Sudan, and obviously the impact on those populations is potentially very serious," he added.

South Sudan -- which declared independence from former civil war enemies in north Sudan in July -- is already reeling from multiple crises, including ethnic clashes, rebel attacks and some 3 million people needing food aid.

Over 80,000 refugees have already fled Sudan's civil war regions of Southern Kordofan and Blue Nile since last June, where Khartoum is battling rebel forces formerly allied to the now independent South Sudanese army.

The UN estimates that more than 500,000 people have been displaced or severely affected by the fighting, but the government, citing security concerns, continues to bar UN and foreign aid workers from the war zone.

The area is at risk of famine if substantial aid deliveries are not delivered by March, the American ambassador to the United Nations, Susan Rice earlier this month, a warning backed by the UN.

The numbers of people arriving into South Sudan seeking aid has at times been as high as at the peak of last year's Horn of Africa crisis, when extreme drought struck the region and famine zones were declared in Somalia.

"In the last week, we had days of influx similar to the peak of influx to Dadaab (refugee) camp in Kenya, so we are speaking of over 1,000 people a day", Da Silva said.

Da Silva said WFP had only until May to pre-position enough food for the rest of the year, before rains in the grossly underdeveloped South made roads impassable.

Over 300,000 people are displaced by internal violence in the South, which also hosts over 100,000 displaced people from Abyei, a border region both sides claim ownership of.