Jean-Pierre Lacroix urged the UN Security Council and the broader international community to ensure that South Sudan's government and opposition return to dialogue and agree on a way forward.
"Both sides claim to be acting in self-defence, while at the same time preparing for the possibility of large-scale hostilities," he said.
UN secretary-general Antonio Guterres condemned the violence saying, "10 million people need life-saving humanitarian assistance & continue to bear the brunt of the conflict".
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 Salva Kiir, an ethnic Dinka, battled those loyal to Riek Machar, an ethnic Nuer.
More than 400,000 people were killed in the war, which ended with a 2018 peace agreement that brought the opponents together in a government of national unity with Kiir as president and Machar as vice president.
But implementation has been slow, and a long-delayed presidential election is now scheduled for December.
In a major escalation of tensions in March 2025, a Nuer militia seized an army garrison.
Kiir's government responded, charging Machar and seven other opposition figures with treason, murder, terrorism and other crimes, and suspended the vice president.
The treason trial has been going on since late 2025.
Lacroix, the UN undersecretary-general for peacekeeping, singled out the escalation of fighting in recent weeks in Jonglei state, northeast of the capital, Juba, pointing to reports of bombings, inflammatory rhetoric, severe restrictions on humanitarian access, and over 280,000 people displaced by the violence.
The peacekeeping chief said the African Union Peace and Security Council, the regional group IGAD and the UN have made clear that there is no military solution and the 2018 peace agreement remains "the only viable framework for peace and stability".
"Let me be clear," Lacroix said, "without consensus, without the participation of all those who have placed their hopes into this peace process, and in all corners, in all 10 states of the country, any election will not be credible and therefore worthy of our support."
He also called South Sudan one of the most dangerous places in the world for humanitarian workers, pointing to 350 attacks on staff and facilities in 2025 compared with 255 in 2024.
The UN 's humanitarian partners report persistent restraints in delivering aid, especially to opposition-held areas, during the country's worst cholera outbreak.
Over 98,000 cases have been reported since it began in September 2024, and there is a resurgence of cases in Jonglei, he said.
Lacroix also cited airstrikes and looting affecting health facilities, most recently a February 3 air attack on a hospital in Lankien, a town in Jonglei, that destroyed critical medical supplies and injured staff.
"These incidents raise serious concerns about shrinking humanitarian space at a time when more than 10 million people require humanitarian assistance, including 7.5 million facing food insecurity and over 1.3 million returnees and refugees from Sudan," he said.
Against this upsurge in fighting, Lacroix said the UN has been forced to cut its peacekeeping force in South Sudan because of a lack of funding, resulting in a reduction in patrols to protect civilians by up to 40 per cent in areas where UN forces are decreasing, and by up to 70 per cent in areas where bases have been forced to close.
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