At least 50 people were killed in this week's clashes between the South Sudan's army and militias in the town of Malakal, UN officials say.
One UN official said 14 civilians and scores of combatants had been killed, while another put the figure at about 50 people altogether.
The army and a militia previously backed by Khartoum exchanged heavy gunfire in the volatile town.
Correspondents say tensions between north and south remain high.
A 21-year civil war ended in 2005 with a peace deal.
But the two sides remain in dispute about oil-rich areas along the border.
Under the deal, South Sudan enjoys considerable autonomy until a referendum is held in 2011 on whether or not the largely Christian and animist south should secede from the Muslim-dominated north.
The Malakal fighting involved supporters of Gabriel Tang, who was backed by the north during the civil war and is now based in Khartoum.
He says he was attacked by the army in the south when he resisted their attempts to arrest him - an accusation also made by the national army spokesman.
The southern army tried to arrest Gabriel Tang
"Provocations against Maj Gen Tang started immediately after his arrival at Malakal, a matter that had obliged him to take refuge with the joint forces, which are the forces that are officially responsible for protecting the people and properties in the town," Brigadier Osman Mohammed al-Aghbash told state media.
But southern Information Minister Gabriel Changson Chang said on Wednesday that the army was seeking to provoke a "new civil war" through the fighting.
Fighting between South Sudan's army and elements in the Tang militia killed 150 people in Malakal in 2006.