The crisis in South Sudan began last week when its president, Salva Kiir, a member of the majority Dinka ethnic group, asserted that he had uncovered a coup and detained 11 people, including former ministers. Mr. Kiir claimed the coup had been backed by former Vice President Riek Machar, who is a member of the rival Nuer ethnic group.
As the violence grew, Mr. Obama sent 45 American troops to protect the embassy in Juba. The United States also organized evacuation flights that have already ferried at least 450 American Embassy personnel, other Americans and some citizens of other nations out of the country.
As attacks have racked South Sudan, however, there has been mounting concern about the safety of 35,000 civilians who have sought sanctuary at United Nations peacekeeping bases, as well as the safety of the peacekeepers themselves.
Armed youths seized a United Nations base in another town, Akobo, on Thursday, killing at least 13 people, including two United Nations peacekeepers.
About 2,000 armed youths have surrounded the United Nations base in Bor. It has been difficult for American officials to get in touch with local rebel commanders there to establish what areas they might control amid signs that the fighting may be escalating into a broader ethnic conflict.
