South Sudanese rebels have taken over a key town, the military has said, as fighting continues after Sunday's reported coup attempt.
"Our soldiers have lost control of Bor to the force of Riek Machar," said army spokesman Philip Aguer.
President Salva Kiir has accused Mr Machar, the former vice-president, of plotting a coup - a claim he denies.
The unrest, which began in the capital Juba, has killed some 500 people and sparked fears of widespread conflict.
Since independence from Sudan, several rebel groups have taken up arms and one of these is said to have been involved in the capture of Bor.
The United Nations has expressed concern about a possible civil war between the country's two main ethnic groups, the Dinka of Mr Kiir and the Nuer of Mr Machar.
In an interview with Radio France Internationale, Mr Machar called on the army to remove the president.
"We want him to leave, that's it," he told the station.
Mr Machar was sacked by Mr Kiir in July.
The UN has called for political dialogue to end the crisis, and the Ugandan government says its president has been asked by the UN to mediate between the two sides.
A delegation of East African foreign ministers has arrived in Juba to try to mediate in the crisis.
The UN peacekeeping mission says it is sheltering civilians in five state capitals, including Juba, Bor and Bentiu, the main town of the oil-producing state of Unity.
Britain and the US have both sent planes to airlift their nationals out of the country, and a US defence official described the situation as "getting ugly".
Bor is the capital of Jonglei state, and even before the current unrest, it was seen as one of the most volatile areas of South Sudan.
Overnight there were reports of gun battles in the town, as renegade officers fought with troops still loyal to the president.
President Salva Kiir has blamed the violence on a group of soldiers who support Mr Machar, saying they tried to take power by force on Sunday night.
South Sudan has struggled to achieve a stable government since becoming independent in 2011.
The oil-rich country remains ethnically and politically divided, with many armed groups active.