The South Sudanese government has lost control of territory north of the capital, as fighting that already has split the armed forces threatens to further divide the country. Civilians are continuing to flee the violence.
Meanwhile, South Sudan's former vice president has called for the overthrow of President Salva Kiir, as tension and violence continue to rise in Africa's newest country.
Riek Machar told Radio France Internationale on Thursday that he would like to see a "palace revolution" in which the military topples the head of state.
The government says soldiers under the command of a renegade general took control of the town of Bor, the capital of Jonglei state, on Wednesday.
Internally displaced persons
The fighting has forced thousands of civilians to flee towns in the area, with many taking refuge at the United Nations compound on the outskirts of the city.
South Sudanese Foreign Minister Barnaba Marial Benjamin told VOA that government officials in Bor were forced to flee the town center for the safety of an army barracks.
“So the fighting is still going. We don’t have full control of it because the army is divided and the fighting continues, destruction continues,” said Benjamin.
Benjamin said the government plans to send additional troops to the area to try to recover control.
“You have to protect the sovereignty of the country. The government forces must make sure that the capital of Jonglei comes back to the sovereignty of the state of South Sudan," said Benjamin.
Fighting broke out between security forces in South Sudan's capital, Juba, earlier this week, prompting widespread violence in the city that, according to U.N. estimates, may have left up to 500 people dead.
Kiir had accused Machar of launching a coup attempt earlier this week with an attack on army headquarters in the capital, Juba. That fighting set off violence that the government says has killed some 500 people and wounded 700 others.
Kiir on Wednesday said he is willing to hold talks with Machar, who he fired as vice president in July during a wider cabinet reshuffle.
Thursday, the government said rebelling soldiers had seized control of Bor, a town north of Juba.
The government insisted it was in total control of Juba, saying the airport had reopened and that government ministries are operating.
However, the U.S. embassy went ahead with an evacuation flight for U.S. citizens who want to leave the country.
Observers have raised concerns that a rift between Machar, from the Nuer ethnic group, and Kiir, a Dinka, could fuel already-chronic tribal violence in South Sudan.
Human Rights Watch Africa analyst Leslie Lefkow said South Sudanese soldiers may have specifically targeted people from the Nuer ethnic group during this week's fighting in Juba.
"We've spoken to a lot of people in Juba who were witnesses of what has been happening over the last few days and people have told us really horrifying accounts of civilians - men, women and children - who were in their houses, in their compounds, hiding from the fighting, and who were actively sought out by soldiers coming into their homes, shooting them, often asking people whether they were Dinka or Nuer," said Lefkow.
Human Rights Watch also said there are reports that Nuer soldiers had targeted ethnic Dinkas.
The government denied there was an ethnic element to what it describes as Machar's "aborted coup."
Peace attempts underway
In another development, top ministers from the regional Intergovernmental Authority on Development [IGAD] group are traveling to South Sudan on a peace mission.
The group was instrumental in mediating a 2005 agreement that ended Sudan's civil war with what was then its southern region.
Kiir said Wednesday he is willing to hold talks with Machar. U.N. Secretary-General Ban Ki-moon has urged the president to engage with his opponents and cooperate with the United Nations.
"This is a political crisis, and urgently needs to be dealt with through political dialogue. There is a risk of this violence spreading to other states, and we have already seen some signs of this," said Ban.
Ban said Wednesday that as many as 20,000 people have take refuge with the U.N. mission in the capital, Juba.
Members of the South Sudan rebel delegation attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Taban Deng Gai, left, head of the rebel delegation and South Sudan's leader of the government delegation, Nhial Deng Nhial, attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Unidentified members of the delegation from the South Sudan government and western observers meet at the Sheraton Hotel, Addis Ababa, Ethiopia, January 4, 2014.
A displaced mother and her baby, one of the few to have a mosquito net, wake up at a refugee camp, Awerial, South Sudan, Jan. 2, 2014.
A young displaced girl carries a bucket of water back to her makeshift shelter at a United Nations compound. The compound has become home to thousands of people displaced by the recent fighting, Juba, South Sudan, Dec. 31, 2013.
Displaced people gather inside a mosquito net tent as they flee from the fighting between the South Sudanese army and rebels in Bor town, in Juba, South Sudan, Dec. 30, 2013.
A displaced woman hangs up laundry on the plastic sheeting wall of a latrine at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
Yared, 2, is held by his mother, Madhn, who fled from the town of Bor a few days ago. She receives medicine for her child at a Medecins Sans Frontieres (MSF) medical tent, at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A young displaced boy rests on the wheel arch of a water truck while others fill containers from it, at a United Nations compound, Juba, South Africa, Dec. 31, 2013.
A family makes tea outside their makeshift shelter at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A general view of a camp for displaced people set up in a United Nations compound in Bor, South Sudan, Dec. 25, 2013.
South Sudan army soldiers hold their weapons as they ride on a truck in Bor, Dec. 25, 2013.