The army in South Sudan is preparing for an offensive to recapture Bor - the only major town still held by rebels.
Troops have been seen boarding barges heading for the frontline.
Government forces re-took the town of Bentiu on Friday.
The BBC says little progress has been reported at peace talks in a hotel in Addis Ababa, Ethiopia.
Rebel leader Riek Machar, a former vice-president, wants 11 prisoners freed before agreeing to peace.
The conflict began on 15 December and has often taken on an ethnic dimension.
South Sudan became independent in 2011 after seceding from Sudan.
