A UN spokesman said the fighting began at daybreak, not far from the town’s UN compound. A South Sudanese army spokesman confirmed a “big fight” had happened.
Tuesday is the deadline given by regional leaders for President Salva Kiir to hold talks with his former deputy, Riek Machar, who is accused of mounting a coup that sparked the violence.
But analysts say there seems to be little chance of the deadline being met.
Only last week, the army was celebrating the recapture of Bor from the rebel forces – but it now appears that the rebels are back, and have captured a key crossroads.
The UN believes the attackers are a mix of mutinous soldiers loyal to Mr Machar and an ethnic militia called the “White Army”, known for putting white ash onto their bodies as a kind of war-paint.
In recent days, thousands of people have fled from Bor, the capital of Jonglei state.