The UN Security Council has urged South Sudan President Salva Kiir to release political prisoners to help bring an end to the bitter conflict there.
Rebel leader, Riek Machar, wants 11 people freed before agreeing to peace.
Mr Machar’s forces appear on the back foot after losing the town of Bentiu to government forces on Friday.
The government says it is mobilising thousands of troops to retake Bor – the last major town controlled by Mr Machar’s forces.
Mr Machar, a sacked vice-president, has expressed determination to hold on to the town, the capital of Jonglei state, some 200 km (125 miles) north of the capital, Juba.
Speaking to the AFP news agency by phone, he sought to explain the loss of Bentiu, capital of the oil-rich Unity State.
“It was to avoid fighting in the streets and save civilian lives,” he said.
Army spokesman Philip Aguer said that fighters on both sides had been killed.
Ahead of the government advance, thousands of people fled Bentiu, while several thousand sought refuge in a UN base in the town.