Khartoum has repeatedly ordered about 40,000 northern SPLA troops, which it says are illegal, to disarm [Reuters]

Fighting in Sudan's volatile oil-producing border state of South Kordofan has left several people dead and wounded, the UN has said.

"We know that more people have been killed overnight this morning in the state capital, Kadugli. But we don't have casualty figures," Hua Jiang, a spokeswoman for the UN mission in Sudan (UNMIS), told the AFP news agency on Wednesday.

Jiang said the fighting, which started on Sunday between Sudanese armed forces and northern elements within the Sudan People's Liberation Army (SPLA), the former southern rebel group, was currently most fierce in Kadugli.

But the army had also been shelling SPLA positions in the mountains of South Kordofan, she added.

"The IDPs (internally displaced persons) are now our main concern," she said.

Ceasefire call

Tension has been escalating in South Kordofan, the north's only oil-producing state, which borders the south.

The state is home to many fighters who sided with the south against the north during decades of civil war and fear being isolated after the south officialy declares independence next month.

The fighters are still referred to as members of the SPLA, although south Sudan says they are no longer part of its army.

Khartoum has repeatedly ordered about 40,000 northern SPLA troops, which it says are illegal, to either disarm or redeploy south of the 1956 borders before southern independence.

The fighting started after northern forces attempted to disarm some of the armed groups, Yasir Arman, who heads the Sudan Peoples Liberation Movement's [SPLM] northern wing, the political wing of the SPLA, said.

"We call for an immediate ceasefire, and to start dialogue immediately," Arman told Reuters.

A spokesman for the northern forces was not immediately available to comment, though Khartoum has previously said the region was stable.

UN staff evacuated

Kouider Zerrouk, another UN spokesperson, said that between 6,000 and 7,000 civilians were in and around the UNMIS peacekeeping compound, near Kadugli's airport, seeking protection.

All UN agencies and international NGOs have suspended their operations in Kadugli since Monday and their staff evacuated to the UNMIS base.

On Tuesday, the UN reported at least six casualties from clashes in the state capital, four of whom where Sudanese police officers.

South Kordofan is awash with weapons and retains strong links to the south, especially among the indigenous Nuba peoples who fought on the side of the southern rebels even though their homeland, the Nuba Mountains, lies in the north.