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Sudan's army has been fighting SPLM-North rebels in the state since June last year, shortly before South Sudan seceded from Sudan, but the South Kordofan capital Kadugli has been mostly isolated from the fighting.

"The government and our troops have been shelling each other since this morning," SPLM-North spokesman Arnu Lodi said. "They have been shelling our forces based outside Kadugli from inside the town and we are shooting back in self-defense."

Sudanese army spokesman al-Sawarmi Khalid could not immediately be reached for comment.

Events in Sudan's border states are hard to verify as the government bans foreign media from travelling there.

Khartoum accuses South Sudan of backing the rebels of the Sudan People's Liberation Movement-North, who fought as part of the southern rebel army during Sudan's civil war, which ended in 2005. South Sudan denies the accusation.

Fighting in South Kordofan and nearby Blue Nile state has displaced or severely affected 900,000 people, the United Nations said on Friday. Sudan agreed in August to let aid into rebel-held areas but the United Nations has been unable to win government approval to go ahead with distributing food.

Under international pressure, Sudan and South Sudan agreed last month to establish a buffer zone along their border after clashing along it several times in the past year. Indirect talks between Khartoum and the rebels, however, have made scant progress.

(Reporting by Khalid Abdelaziz; Writing by Ulf Laessing; Editing by Kevin Liffey)