South Sudan rebels have taken over the town of Bor, capital of Jonglei state, as fighting continues after allegations of a coup d'état on 15 December.
At least 19 civilians have been killed in Bor, according to the South Sudan Red Cross. Thousands have fled the town, seeking shelter at the UN Mission in South Sudan (UNMISS) bases, to which another 20,000 refugees had fled.
The authorities in Bor were not answering their phones, leading the central government to believe they had defected, said Philip Aguer, the South Sudanese military spokesman.
"There was shooting last night...we don't have information on casualties or the displaced in the town, as operations are ongoing," Mr Aguer told reporters.
The clashes began in Juba, capital of South Sudan, after soldiers loyal to former vice-president Riek Macha were accused of attempting to overthrow the regime. Mr Mache denied the coup and any link with the fighting, and accused President Salva Kiir of "inciting tribal and ethnic violence".
"Salva wanted to frame me. I had to flee. They are hunting me down."Mr Mache told BBC.
Although the situation in Juba has since calmed, violence appears to be spreading to other parts of the country.
Tensions have been accumulating in South Sudan since Kiir fired Machar in July. The continuous ethnic clashes between the majority Dinka tribe of President Salva Kiir, against those from the ousted vice-president Riek Machar's Nuer ethnic group, are sparking fears of a civil war.
The fighting has already killed about 500 people, according to UN diplomats, and at least 800 more have been wounded.
UN secretary general Ban Ki-moon is deeply concerned about the escalating violence in South Sudan, and he has called for political dialogue.