The United Nations provided sobering details on Friday of an assault on a peacekeeping base in South Sudan that underscored the organization’s fragile ability to protect civilians in the country, where sectarian mayhem has escalated in recent days.

Two thousand armed youths of Nuer ethnicity overran the facility the previous day, killing at least 11 ethnic Dinka civilians seeking refuge and two Indian peacekeepers who had tried to protect them, the United Nations said.

The assault on the base in the town of Akobo in Jonglei State on Thursday was among a number of alarming developments that have increased the fear of an impending civil war in South Sudan, the world’s youngest country, where ethnic hatreds, initially obscured after independence from Sudan two years ago, have been reaggravated and inflamed.

Between 35,000 and 40,000 civilians have taken refuge in at least three other United Nations peacekeeping bases in the country, including two in the capital of Juba, United Nations officials said, and there were fears of an attack on a peacekeeping base in Bor, the capital of Jonglei State, where 14,000 civilians had sought sanctuary in a base surrounded by at least 2,000 armed youths.