"The food security situation has worsened as early warnings highlight drought conditions in parts of Somalia," a report from the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
"Aid organisations have been unable to meet the needs of over 350,000 estimated displaced people in Mogadishu," it added, warning of "alarming malnutrition rates" in the capital.
The notice comes three years after more than 250,000 people, half of them children, died in the devastating 2011 famine.
But reports of a hunger crisis inside the capital casts a further pall over the government's record, following accusations of corruption as well as continued attacks by Islamist Shebab insurgents against even the most fortified areas.
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The report blamed "funding shortages and a volatile security situation, which has at times restricted aid delivery into the settlements".
Earlier this month the UN warned Somalia is sliding back into an acute hunger crisis, with parts of Mogadishu facing emergency levels just short of famine.
UN assessments also show "above emergency levels of malnutrition" in seven towns, including the major centres of Garowe, Galkayo and Kismayo, with the "highest deterioration" reported in Mogadishu.
Thousands displaced by war live in basic makeshift shelters in Mogadishu, where Al-Qaeda-linked Shebab fighters continue to launch regular attacks, including brazen commando raids on the presidential palace and parliament.
The hardline Shebab once controlled most of southern and central Somalia, including large parts of the capital, but have been driven out of positions in Mogadishu and Somalia's major towns by a 22,000-strong African Union force.