Calling the situation unfolding in the country "dire," Nicholas Haysom said international efforts to broker a peaceful solution can only succeed if President Salva Kiir and his rival-turned-vice president, Riek Machar, are willing to engage "and put the interests of their people ahead of their own".
There were high hopes when oil-rich South Sudan gained independence from Sudan in 2011 after a long conflict. But the country slid into a civil war in December 2013 largely based on ethnic divisions when forces loyal to Kiir, an ethnic Dinka, battled those loyal to Machar, an ethnic Nuer.
More than 40,000 people were killed in the war, which ended with a 2018 peace agreement that brought Kiir and Machar together in a government of national unity.
Elections were supposed to be held in February 2023, but they were postponed until December 2024 — and again until 2026.
The latest tensions stem from fighting in the country's north between government troops and a rebel militia, known as the White Army, believed to be allied with Machar.
Earlier this month, a South Sudanese general was among several people killed when a United Nations helicopter on a mission to evacuate government troops from the town of Nasir, the scene of the fighting in Upper Nile state, came under fire.
Days earlier on March 4, the White Army overran the military garrison in Nasir and government troops responded by surrounding Machar's home in the capital, Juba, and arresting several of his key allies.
Haysom said tensions and violence were escalating "particularly as we grow closer to elections and as political competition increases, sharpens between the principal players".
"Rampant misinformation, disinformation and hate speech is also ratcheting up tensions and driving ethnic divisions, and fear," Haysom said.
"Given this grim situation," he said, "we are left with no other conclusion but to assess that South Sudan is teetering on the edge of a relapse into civil war."
Haysom, who heads the nearly 18,000-member UN peacekeeping mission in South Sudan, warned that a relapse into open war would lead to the same horrors that ravaged the country, especially in 2013 and 2016.
