Heavy fighting in Sudan's war-torn border region saw rebels loyal to an outlawed political party drive government forces out of a key garrison town, the party's secretary general said on Monday.

Yasser Arman also demanded that the government release more than 140 leaders of the northern branch of the Sudan People's Liberation Movement (SPLM-N), the country's main opposition party, which was banned earlier this month.

And he accused Khartoum of "war crimes" for denying humanitarian assistance to Blue Nile and South Kordofan, Sudan's two conflict states that both border the newly independent south and are SPLM-N strongholds.

"Aerial bombardments, which are mainly targeting the civilian population in Blue Nile and South Kordofan, are continuing," Arman told AFP by phone from London.

"Yesterday (Sunday), the SPLM was attacked by the Sudanese armed forces near Talodi. There was heavy fighting and the army was repulsed by the SPLM-North.

"As a result, the government lost three outposts at their garrison in Talodi. Our forces are now besieging Talodi," Arman said.

He gave no details of casualties.

Army spokesman Sawarmi Khaled Saad denied there had been any serious fighting in Talodi, saying only that the army had retaliated after the SPLM tried to "disrupt the security" in the area.

The conflict first erupted in South Kordofan in June, just one month before South Sudan's formal declaration of independence from the north, after disputed local elections between the ruling party in Khartoum and the SPLM-North.

The army's insistence on expelling or forcefully disarming SPLM elements is what appears to have triggered the fighting, which spilled into nearby Blue Nile state at the beginning of September.

Shortly afterwards, President Omar al-Bashir declared a state of emergency in Blue Nile and sacked the SPLM-North governor, while the party was shut down two days later and dozens of its members arrested.

An SPLM-North source in South Kordofan said earlier that the army had bombed a village in Buram county on Thursday, killing three teenage girls.

It has been virtually impossible to get independent confirmation of the violence in South Kordofan and Blue Nile, with the UN peacekeeping mission disbanded in July and international NGOs denied access.

On Monday, Sudan's Vice President Ali Osman Taha flew to Kadugli, the capital of South Kordofan, to attend the opening of the state assembly, where more than 50 members of the SPLM-North were absent.

He urged the rebels to lay down their weapons and the absent members to return.

"Everyone who puts down their weapons and comes back will be welcome. And everyone who comes back to his seat in this parliament will be welcome," Taha said in a speech broadcast live on state television.

"But those who insist on imposing their agenda by fighting... the government forces are ready for them," he added.