Sudanese rebels on Friday denied the military had regained control of a town in Blue Nile state, where authorities allege a months-long insurgency is backed by South Sudan.

The Sudanese Media Centre (SMC), which is close to the security apparatus, reported late Thursday that Sudan's army was again in control of Bau, about 50 kilometres (30 miles) southwest of the state capital Ed Damazin.

Sudan and the South face a UN-imposed Friday evening deadline to cease all hostilities under a resolution which also calls on both sides to stop supporting rebel groups against each other.

SMC said the state governor Al-Hadi Bushra earlier on Thursday addressed troops who "liberated" the town in another victory "against southern Sudan invaders" who had already been forced from Sudan's main Heglig oil field.

Arnu Ngutulu Lodi, spokesman for the rebel Sudan People's Liberation Movement-North (SPLM-N), denied Bau had been taken.

"They entered but they were repelled," he told AFP, but was unable to immediately give more details.

The SPLM-N says it seizes weapons from the Sudanese military and denies Khartoum's allegations that it is supported by South Sudan. The South in turn says Khartoum backs militias on its side of the border.

Sudan has cited security concerns in severely controlling access for foreign relief agencies to Blue Nile and South Kordofan, another border state where a similar conflict began last year. A surging number of hungry refugees are fleeing the fighting, the UN said this week.

Journalists are not permitted to report freely in the area.

The United Nations said last Monday that sporadic fighting had been reported earlier in April in the Bau area, where government troops reportedly took control of Magaja village southwest of Bau town.

"The impact of fighting on civilians in the area was not possible to ascertain due to the lack of access," the UN said.

Lodi said earlier there had been an upsurge in fighting in Blue Nile since border clashes between Sudan and South Sudan escalated in April with waves of air strikes hitting the South, and Southern forces seizing the north's Heglig oil hub on April 10.

After a 10-day occupation the South said it withdrew from Heglig but Khartoum said its forces drove out the South Sudanese.

Ethnic insurgents of the SPLM-N fought alongside southern rebels during Sudan's 22-year civil war, which ended in a 2005 peace deal and the South's independence last July.

Fighting began in September between the army and forces loyal to the elected SPLM-N Blue Nile governor Malik Agar.