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, several rebel groups have taken up arms and one of these is said to have been involved in the capture of Bor.
The UN 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.
The United Nations 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.
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”.
On Wednesday the mayor of Bor, Nicholas Nhial Majak, told the BBC’s Focus on Africa programme that violence had spread there from Juba, 200km (125 miles) away.
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.
“There was shooting last night… we don’t have information on casualties or the displaced in the town, as operations are ongoing,” Col Aguer told reporters.
South Sudan violence, explained in 60 seconds
The army said Peter Gadet Yak – the commander of Division 8 unit – had rebelled, taking with him an unknown number of soldiers.
BBC Africa security correspondent Moses Rono says it is not immediately clear if troops loyal to Mr Machar are working together with them, or if in fact they are the same soldiers.
But he says this is likely, because of the history of relations between the two, and also the fact they are both from the Nuer community.
Also, in 1991, when Mr Machar broke off from the Sudan People’s Liberation Movement (SPLM), which now governs South Sudan, Mr Gadet went with him.
This latest violence is not confined to Jonglei.
Tensions are also said to be high in the states of Unity and Upper Nile, but in Juba – where the violence started – the situation appears to be calmer, with Col Aguer saying “the streets are busy and shops are open”.
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.
But Mr Machar denied allegations that he had tried to stage a coup, telling the BBC: “Salva wanted to frame me. I had to flee. They are hunting me down.”
He blamed Sunday’s fighting on a conflict between members of the presidential guard, and added that government troops had used the incident to arrest some of his supporters.
South Sudan has struggled to achieve a stable government since becoming independent from Sudan in 2011.
The oil-rich country remains ethnically and politically divided, with many armed groups active.
After a peace deal was signed in 2005, Mr Machar was appointed vice-president of the South Sudan regional government.
He retained the position after independence in 2011 but was dropped in July when the whole cabinet was sacked.