A political crisis in South Sudan, the world’s youngest country, worsened significantly on Thursday, as a deadly assault hit a United Nations peacekeeping base, the number of civilians seeking refuge in the organization’s other facilities there exceeded 30,000 and diplomats expressed fears about the potential for a civil war.

Britain began evacuating its citizens on Thursday. The United States suspended operations at its embassy in Juba, the capital, this week and strongly advised Americans to leave.

In New York, Jan Eliasson, the United Nations deputy secretary general, said that the peacekeeping force’s base in the town of Akobo in Jonglei State had been assaulted, “and we have reports that lives are lost.” Later in the day, India’s United Nations ambassador, Asoke Mukerji, said three Indian peacekeepers in Akobo had been killed.

Kieran Dwyer, a spokesman for the United Nations Department of Peacekeeping Operations, said late Thursday that communications with the base in Akobo had been lost. “We have had some communications with peacekeepers in the area subsequent to the attack, but communications are extremely difficult, and we need to get our teams in by helicopter first thing tomorrow morning before we will know more,” he said in an email.