"The level of malnutrition, especially among children, is of serious concern, with nearly 305,000 children under the age of five years acutely malnourished," said UN aid chief for Somalia Peter de Clercq. "We estimate that 58,300 children face death if they are not treated."
The grim assessment, based on the latest data collected by the UN, comes just over four years since intense drought and war sparked a famine killing more than a quarter of a million people.
Floods and failed rains caused by the El Nino weather phenomenon have sparked a dramatic rise in the number of people going hungry in large parts of Africa.
Northern Somali areas, including self-declared independent Somaliland along the Gulf of Aden and semi-autonomous Puntland, are especially hard hit.
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"The food security and malnutrition situation in Somalia is alarming, especially in parts of Puntland and Somaliland, which have been hard hit by drought exacerbated by El Nino," the UN said.
The UN is calling for $885 million (792 million euros) in aid.
Fighting continues between Somalia's Al-Qaeda linked Shebab insurgents and government and allied forces, backed by the African Union force, which counts more than 20,000 members.
The warning comes as neighbouring Ethiopia struggles to combat its worse drought for 30 years.