* Army says five killed in earlier shelling
* Kadugli has been mostly isolated from fighting (Adds army comment, background)
KHARTOUM, Oct 10 (Reuters) - Insurgents said they had shelled the main city in Sudan's oil-producing South Kordofan state on Wednesday, the second attack on the city this week.
Sudan's army has been battling SPLM-North insurgents in South Kordofan since June last year, shortly before South Sudan, which adjoins the state, seceded. But the capital Kadugli has largely been spared so far.
Khartoum accuses South Sudan of backing the rebels, an accusation that it denies.
"We are acting in self-defence. 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."
A local witness told Reuters that four shells had landed in an eastern suburb of Kadugli on Wednesday morning after some shelling of the district the previous night. No casualties were reported.
Sudanese army spokesman Al-Sawarmi Khalid denied the army had bombed any rebel positions and said he could not confirm any new shelling.
"The armed forces did not register any shelling or attack on Kadugli last night or today," he said.
The rebels say they began to attack Khartoum's forces in Kadugli only after their own positions were bombed.
Sudanese state media said five people had been killed by SPLM-North artillery on Monday. The United Nations said at least one rocket had hit a U.N. compound in Kadugli, and moved out its staff, complaining of a violation of humanitarian law.
Fighting in South Kordofan has forced hundreds of thousands to flee their homes and added to tensions between Sudan and South Sudan, former enemies in a civil war that ended in 2005.
The two countries agreed last month under international pressure to establish a buffer zone after clashing along their border several times in the past year.
However, there has been scant progress in parallel indirect talks between Khartoum and the rebels of the Sudan People's Liberation Movement-North, who fought as part of the southern rebel army during the civil war. (Reporting by Ulf Laessing, Khalid Abdelaziz and Alexander Dziadosz; Editing by Kevin Liffey)