The United Nations said the incident, which could deal a blow to an already fragile peace process, was "utterly abhorrent" and a possible war crime.

The aircraft came under fire as it tried to rescue wounded soldiers, the United Nations said. The attack came amid fears that a critical power-sharing deal was at risk of collapse, risking all-out civil war.

NAIROBI - A United Nations helicopter attempting to evacuate South Sudanese troops came under fire in the northern town of Nasir on Friday, the U.N. mission there said, resulting in the death of a crew member and several soldiers including a general.