UNITED NATIONS -- The United Nations says youth from South Sudan's second-largest ethnic group have forced their way into a U.N. peacekeeping mission base in conflict-wracked Jonglei state, pursuing civilians who had taken refuge there.
U.N. deputy spokesman Farhan Haq said Thursday the U.N. fears "there may have been some fatalities but can't confirm who and how many at this stage."
Rapidly escalating ethnic violence has raised fears of instability in the world's newest country.
Haq says the situation in the capital, Juba, appears to have calmed to some extent, but he noted unconfirmed reports of several students killed by security personal at Juba University on Wednesday.
He says thousands have sought refuge and called for U.N. protection.
The government no longer controls the capital of an oil-producing state, officials said Thursday.
A ruling party official also said the president's earlier claim that an attempted coup had triggered the ongoing violence was false. Instead, it erupted Sunday when the presidential guard attempted to disarm fellow guard members who belong to the Nuer tribe, said Choul Laam, chief of staff for the secretary general 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, Laam told reporters in Nairobi.
The South Sudan government has said the violence has already killed up to 500 people. Juba, the capital, was reported calm there on Wednesday and Thursday, but clashes were reported in Jonglei state.
"The situation in South Sudan can be best described as tense and fragile. If it is not contained it could lead to ethnic cleansing," 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," 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 U.N. Secretary-general's office, citing figures from the South Sudan Red Cross.
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 lawmaker from Jonglei, said soldiers loyal to Machar now control Bor.
Machar, an influential politician who is a hero of the brutal war of independence against Sudan, is Kiir's rival for top leadership of the ruling Sudan People's Liberation Movement party. Tensions had been mounting since Kiir fired Machar as his deputy in July. Machar, the deputy chairman of the ruling party, later said he would contest the presidency in 2015.
The think tank Eurasia Group said Kiir's firing of Machar in Julyalienated the "long aggrieved Nuer" in a country with "a factionalized military and a history of violent ethnic rivalries."
United Nations chief Ban Ki-moon told reporters Wednesday that South Sudan was experiencing a political crisis that "urgently needs to be dealt with through political dialogue." Ban said he urged 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.