A crew member was killed and two others were injured when a United Nations helicopter on an evacuation mission in a remote part of South Sudan was attacked.

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.