The top U.N. official in South Sudan is warning that the country is teetering on the edge of renewed civil war.

A power-sharing deal between Kiir and Machar has been gradually unravelling, threatening a return of the civil war that killed around 400,000 people between 2013 and 2018.

An attack on a U.N. helicopter has highlighted rising tensions in the world’s youngest country, where a seven-year-old peace ...