Famine has been declared in South Sudan, with a million people said to be at risk of starvation.
Unicef, the World Food Programme and the Food and Agricultural Organisation said on Monday that 100,000 people are "already starving", while a further one million are classified as being on the brink of famine.
They added that 5.5 million people, half the population, are considered to be at risk of food shortages by summer.
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It is the first time in six years that famine has been announced anywhere in the world, Sky News reports.
The aid agencies said the situation in South Sudan was the "worst hunger catastrophe since fighting erupted more than three years ago".
Serge Tissot, from the UN's Food and Agriculture Organisation, said: "Our worst fears have been realised. Many families have exhausted every means they have to survive."
UN officials have accused the South Sudanese government of blocking food deliveries to some of the worst affected areas.
New York Times East Africa correspondent Jeffrey Gettleman told NPR that aid convoys and warehouses have been attacked "both by the government and by the rebels".
The wider effects of the civil war are at the root of the problem, with the conflict disrupting food distribution networks, destroying agricultural land and displacing the country's farmers.
While South Sudan greeted its independence from Sudan in 2011 with jubilation, the country's complex ethnic politics then turned extraordinarily violent and the last three years have seen it descend into intense in-fighting.
Government forces have been accused of brutal reprisals following rebellions by the Shilluk and Murle minorities, while tens of thousands of men and child soldiers belong to informal militias accused of mass executions, rape and torture of rival ethnic groups.
The UN has warned that South Sudan is the country at the highest risk of experiencing a genocide.
Joyce Luma, from the UN's World Food Program, said aid agencies are trying to fend off mass starvation in South Sudan "with all our might", but warned: "There is only so much that humanitarian assistance can achieve in the absence of meaningful peace and security."
