Hundreds have been killed and injured in clashes with George Athor's men this year
At least 50 people have reportedly been killed in South Sudan clashes, as rebel leader George Athor claims a new militia has joined him.
Both Mr Athor and South Sudan's army say more than 50 people have died in clashes in Upper Nile State but each says most of the casualties were on the other side.
The claims have not been verified.
The increasing violence comes as South Sudan prepares to declare independence from the north in July.
An overwhelming majority of voters backed secession in January's referendum - part of a peace deal following decades of north-south conflict.
President Omar al-Bashir has said he accepts the independence of the oil-rich south.
His advisers deny southern claims that they are linked to Mr Athor's fighters.
"We hope the south will be strong - this will reflect stability in the north," Rabie Abd al-Atti, a senior official with Mr Bashir's National Congress Party, told the BBC.
But southern army spokesman Col Phillip Aguer said weapons and munitions had been seized from a militia led by a man called Oliny in Upper Nile state, which came from the north.
Mr Athor told the BBC that Mr Oliny's fighters had joined his rebellion.
He also said his forces had been attacked by the army in Jonglei State.
Fighting in Jonglei between Mr Athor and the southern army has reportedly left hundreds dead this year.
The BBC's James Copnall in Khartoum says Mr Athor's rebellion is one of the great challenges facing the southern authorities as they move towards secession.
If it is not stamped out quickly it may well encourage others to take up arms, our correspondent says.
Mr Athor was a senior member of the Sudan People's Liberation Army during the war against the north.
He turned renegade last April, claiming he was cheated out of victory in elections for the post of state governor of Jonglei.
Despite a ceasefire agreement in January, clashes broke out again in Jonglei in February.