Hundreds of rebels have been killed in Sudan's South Kordofan state following clashes with the army, officials have said - claims denied by the rebels.
Governor Ahmed Haroun said the SPLM-North rebels were killed when the army repelled an assault on Talodi city.
But a rebel spokesman said fighting was continuing and "many" government troops had been killed.
Talodi lies near the border with South Sudan, which became independent from Sudan in July.
The BBC's James Copnall in Khartoum says capturing the strategic city of Talodi would be a major success for the Sudan People's Liberation Movement-North rebels.
He says they hold a considerable amount of territory in South Kordofan, though largely in difficult to reach mountain areas.
It is also fighting the government in Blue Nile state, but our correspondent says this is a more conventional war which is easier for Khartoum.
"Hundreds of soldiers from the Sudan People's Liberation Movement (SPLM-North) were killed during an attack on the city of Talodi this morning," Mr Haroun said.
Mr Haroun has been indicted by the International Criminal Court for alleged war crimes in Darfur, where he used to be governor, and our correspondent says he is feared by the SPLM-North rebels.
Human rights groups have accused the army of carrying out ethnic cleansing in South Kordofan against the Nuba people, seen as pro-SPLM-North.
Sudanese army spokesman Sawarmi Khaled Saad said more than 700 rebels attacked Talodi, east of the state capital Kadugli, the AFP news agency reports.
"The armed forces waited for the invaders to arrive on three fronts with equipment and on several vehicles, but in an hour the armed forces and popular defence forces beat back the attack, causing heavy losses," he is quoted as saying.
But an SPLM-North spokesman told the BBC that just two rebel fighters had been killed.
South Kordofan is one of three border areas - along with Abyei and Blue Nile - to have been affected by conflict since South Sudan became independent.
Tens of thousands of people have fled fighting in the three areas.
Sudan lodged a complaint with the UN Security Council in August, accusing South Sudan of backing the rebels.
The SPLM, in power in South Sudan, denies Khartoum's claims, even though it fought alongside the northern rebels during Sudan's decades-long civil war.
South Sudan, meanwhile, accuses Khartoum of backing the South Sudan Liberation Army rebels which launched attacks in Unity state, across the border from South Kordofan, on Saturday, killing at least 75 people.
But SSLA rebel leader Major General Bapiny Montyuil denied the claims in a BBC interview.
Unity state is rich in oil, while South Kordofan is Sudan's only oil-producing region, following the south's independence.