South Sudan’s military said on Thursday it no longer controlled a key town in a rural state where fighting has spread in the aftermath of what the government says was an attempted coup mounted by soldiers loyal to a former deputy president.

The authorities in Bor, the capital of the state 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,” he said.

At least 19 civilians have been killed in violence in Bor, Martin Nesirky, a spokesman for the United Nations secretary-general’s office, said on Wednesday, citing figures from the South Sudan Red Cross. He said tensions were also on the rise in the states of Unity and Upper Nile.

Ethnic rivalry is threatening to tear apart the world’s newest country, with the clashes apparently pitting soldiers from the majority Dinka tribe of President Salva Kiir against those from ousted Vice-president Riek Machar’s Nuer ethnic group. At least 500 people, most of them soldiers, have been killed in violence since the alleged coup attempt on Sunday, the government said on Wednesday. At least 700 more have been wounded, Information Minister Michael Makuei Lueth said.

Although Juba, the South Sudanese capital where the alleged coup was mounted, has since become calm, violence appears to be spreading to other parts of the oil-rich East African nation.

Tensions have been mounting in South Sudan since Mr. Kiir fired Mr. Machar as his deputy in July. Mr. Machar has said he will contest the presidency in 2015.

Mr. Machar himself is the subject of a manhunt by the country’s military after he was identified by Mr. Kiir as the leader of an alleged coup attempt on Sunday. Mr. Machar has denied he was behind any coup attempt.

Mr. Kiir told a news conference in Juba late on Wednesday that he was willing to enter talks with Mr. Machar, a rival for power within the ruling Sudan People’s Liberation Movement party.

U.N. chief Ban Ki-moon told reporters on Wednesday 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.

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