KHARTOUM (Reuters) - Insurgents said they shelled the main city in Sudan's oil-producing South Kordofan state near the border with South Sudan on Wednesday, the second time this week.

Sudan's army has been battling SPLM-North insurgents 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.

"We are acting in self-defense. The Sudanese army has been bombing our positions with Antonov planes around Kadugli since yesterday," SPLM-North spokesman Arnu Lodi said. "We are shelling military positions in Kadugli."

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

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

Sudanese state media said five people were killed when the SPLM-North shelled Kadugli on Monday. At least one rocket hit a U.N. compound in Kadugli, according to the United Nations.

The rebels said they began the first assault on the central government stronghold since last year only after Khartoum's army bombed their positions.

Fighting in South Kordofan has forced hundreds of thousands of people to flee their homes and stoked tensions between Sudan and South Sudan since they split apart last year under a treaty that ended the long civil war.

Under international pressure, Sudan and South Sudan agreed last month to secure 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 Ulf Laessing and Khalid Abdelaziz; Editing by Mark Heinrich)