South Sudan's government has lost control of the capital of an oil-producing state amid a bloody unpheaval which has killed hundreds of people.
A ruling party official said the president's earlier claim that an attempted coup had triggered the fighting was false.
Instead, the violence erupted on Sunday when the presidential guard attempted to disarm fellow guard members who belong to the Nuer tribe, said Choul Laam, of the ruling Sudan People's Liberation Movement.
Those who tried to do the disarming were members of the majority Dinka tribe of President Salva Kiir, Mr Laam told reporters in Nairobi.
The South Sudan government has said the violence has killed up to 500 people. The fighting started in Juba, the capital, and while it was reported calm there on Wednesday and today, clashes were reported in Jonglei state as ethnic rivalry and a power struggle between the president and his ousted deputy strained the seams of the world's newest country.
"The situation in South Sudan can be best described as tense and fragile. If it is not contained it could lead to ethnic cleansing," Mr Laam said. The International Crisis Group reported that armed groups in Juba have "targeted civilians based on ethnicity".
Authorities in Bor, the capital of Jonglei, were not answering their phones, leading the central government to believe they had defected, said Philip Aguer, the South Sudanese military spokesman.
"We lost control of Bor to the rebellion," Mr Aguer said.
He said there were reported gunfights in Bor overnight as renegade officers tried to wrest control of the town from loyalist forces. At least 19 civilians had been killed there, said Martin Nesirky, a spokesman for the UN Secretary-general's office.
President Kiir had announced to the nation that the violence was started by an attempted coup led by ousted Vice President Riek Machar. Machar, a Nuer, has denied he was behind any coup attempt. Jodi Jongole Boyoris, a politician from Jonglei, said soldiers loyal to Mr Machar now control Bor.
Mr Machar, an influential politician who is a hero of the brutal war of independence against Sudan, is Mr Kiir's rival for top leadership of the Sudan People's Liberation Movement party. Tensions had been mounting since Mr Kiir fired him as his deputy in July. Mr Machar, the deputy chairman of the ruling party, later said he would contest the presidency in 2015.
The think tank Eurasia Group said the firing of Mr Machar alienated the "long aggrieved Nuer" in a country with "a factionalised military and a history of violent ethnic rivalries."
United Nations chief Ban Ki-moon told reporters that South Sudan was experiencing a political crisis that "urgently needs to be dealt with through political dialogue." Mr Ban said he urged Mr Kiir "to resume dialogue with the political opposition."
South Sudan has been plagued by ethnic violence since it peacefully broke away from Sudan in 2011 after decades of civil war.