The entire population of the Heglig area has fled a military standoff between Sudan and South Sudan, leaving thousands of civilians displaced without shelter, the United Nations has said.

"According to the government of Sudan's Humanitarian Aid Commission [HAC] and other reports received by the UN, the entire civilian population of Heglig town and neighbouring villages fled," the UN's humanitarian agency said on Sunday.

The report cited HAC figures as saying 5,000 people had escaped from Heglig, mostly to the communities of Kharasana and Keilak, about 100km north of Heglig, Sudan's main oilfield.

South Sudan's army said on Sunday that it had completed its withdrawal from Heglig, but condemned the north for bombing the area as the pullout was still under way.

Heglig is part of South Kordofan state, where Sudan has cited security concerns in severely controlling access for foreign relief agencies.

South Sudan occupied the field on April 10, in a move that coincided with waves of air strikes against the South, sparking fears of wider war. Khartoum accuses the South of backing rebel groups in the border state, an allegation Juba denies.

A Sudanese Red Crescent Society (SRCS) mission to Kharasana and Keilak "reported that most of the displaced people are scattered in the bushes or living in the open approximately three to four kilometres away from Kharasana town," said the UN report covering the period until April 15.

"The SRCS mission indicated that food, water and sanitation are priority needs for the displaced in Kharasana," the UN said.

'Dozens killed'

Sudan also said on Sunday that it had repulsed a "major" rebel attack on a strategic town in its South Kordofan state, the latest outbreak of violence in its volatile border area with South Sudan.

Rebels had launched an attack on Talodi, a border town the rebels have repeatedly tried to seize, said Al-Sawarmi Khalid, Sudanese army spokesperson.

"Dozens of the rebels were killed. The army is expelling the remaining rebel forces," he said.

In-depth coverage of North-South strife over border

Rebels have been fighting in South Kordofan and nearby Blue Nile for months.

Recent tensions between Sudan and South Sudan have been fueled by a dispute over how much the landlocked South should pay to export oil via pipelines and other infrastructure in Sudan.

Juba shut down its roughly 350,000 barrel-a-day output in January, accusing Sudan of seizing some of its crude.
Oil accounted for about 98 per cent of the South's state revenues.