Rebels in Sudan's South Kordofan have captured a key government outpost near the state capital, a rebel spokesman said, but the country's armed forces denied even being in the area.

The battle took place on Sunday at El-Ahmier, 30 kilometres (19 miles) southeast of Kadugli, said Arnu Ngutulu Lodi of the Sudan People's Liberation Movement-North (SPLM-N).

"It was the last outpost of SAF south of Kadugli," he told AFP, referring to the Sudanese armed forces.

Speaking from Kenya, Ngutulu said he had not yet received details of casualties, but the rebels had captured vehicles including an ambulance, as well as mortars, heavy and light machineguns and an anti-tank missile.

"We didn't have any troops in El-Ahmier", said Sudanese army spokesman Sawarmi Khaled Saad, denying that weapons were seized.

Lodi said fighting had also resumed over the past two days further south at Taruje near the disputed southern border.

Access to the region is restricted, making independent verification difficult.

The SAF on Saturday denied bombing civilians in an operation which rebels said left 16 villagers and five government troops dead in an area about mid-way between El Ahmier and Taruje.

Oil-producing South Kordofan remained under Khartoum's administration when South Sudan became independent in July, but fighting since June has pitted ethnic Nuba rebels, once allied to rebels in the South, against the Sudanese army.

A similar conflict is going on in nearby Blue Nile state.

The United Nations says hundreds of thousands of people have become internally displaced or otherwise severely affected because of fighting in the two states.

Refugees fleeing to Ethiopia and South Sudan are reporting food shortages and rising levels of malnutrition, with "particularly alarming" signs coming from rebel-held areas, Valerie Amos, the UN's top emergency aid official, said during a visit to Khartoum this month.

The government, citing security concerns, continues to bar UN and foreign aid workers from the warzone.

South Kordofan's governor, Ahmad Harun, is wanted by the Hague-based International Criminal Court for alleged war crimes and crimes against humanity committed in the Darfur region of western Sudan.

Sudan and South Sudan, which became independent last July in a referendum that followed two decades of civil war, have accused each other of supporting rebels inside their respective borders.