The United Nations has officially declared a famine in South Sudan – the first to be declared anywhere in the world in the past six years.

Unlike most other famines, the starvation in South Sudan is not a result of drought or natural disaster. This one is a man-made catastrophe, caused by war and economic collapse, the UN says.

Three UN agencies announced on Monday that they have declared a famine in a region of South Sudan where an estimated 100,000 people are facing starvation. A further one million people are on the brink of famine as war rages on in the world's newest country, they said.

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The agencies described how destitute farmers are scrounging for wild plants because their animals are dead, while urban dwellers are facing disaster because of hyperinflation, with prices soaring up to 800 per cent in the past year.

"A formal famine declaration means people have already started dying of hunger," the UN agencies said in a joint statement. "Urgent action is needed to prevent more people from dying."

More than 250,000 children are already severely malnourished and at risk of death, the agencies said. In some areas, as much as 42 per cent of the population is acutely malnourished, they said.

It is the first famine to be declared anywhere since the Somalia famine of 2011. The latest famine is centred in Unity State in the north-central part of South Sudan. For a famine to be officially declared, the UN must decide that the situation has exceeded a specific threshold of malnutrition and death.

"This famine is man-made," said Joyce Luma, the South Sudan director of the UN World Food Program.

"WFP and the entire humanitarian community have been trying with all our might to avoid this catastrophe," she said in a statement. "But we have also warned that there is only so much that humanitarian assistance can achieve in the absence of meaningful peace and security."

The three UN agencies – the WFP, the UN Children's Fund (UNICEF) and the Food and Agriculture Organization (FAO) – are now estimating that urgent food assistance is needed by 4.9 million people, more than 40 cent of South Sudan's entire population. This could rise to 5.5 million people by July if there's not enough aid, they said.

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South Sudan has been in the grip of a brutal civil war for more than three years, with tens of thousands already dead. But the current food crisis is the most dangerous since the war erupted, the agencies said.

"Our worst fears have been realized," said Serge Tissot, the FAO representative in South Sudan.

"Many families have exhausted every means they have to survive," he said. "The people are predominantly farmers and war has disrupted agriculture. They've lost their livestock, even their farming tools. For months there has been a total reliance on whatever plants they can find and fish they can catch."

Civil war began in the oil-rich country in 2013, just two years after it gained independence from Sudan. Peace agreements and ceasefires have been repeatedly ignored, and investigators have documented a wide range of atrocities, including massacres, gang rapes, beheadings, ethnic cleansing and forced cannibalism. Relief workers and doctors have been among the thousands of civilians killed by rampaging militias and government forces.

"The declaration of famine in parts of South Sudan is just the final step on a long path of hunger and conflict," said Fred McCray, the South Sudan director for CARE, the international relief agency.

"The last three years of violence have pushed people over the edge of survival, leaving many families with nothing but leaves and roots to eat."

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Three years of war have "severely undermined" crop production in South Sudan, the UN agencies said. They found an acute malnutrition rate above 15 per cent – the emergency threshold – in 14 of the 23 counties that they assessed.

"The upsurge in violence since July, 2016, has further devastated food production, including in previously stable areas," the UN agencies said. "Urban populations are also struggling to cope with massive price increases on basic food items."

In the world's last famine, more than a quarter of a million people died in Somalia. Aid agencies have warned that Somalia is at risk of famine again this year, with the same warning signs as in the lead-up to the 2011 famine.

Relief agencies have struggled to find enough resources for the many crises across the world in recent months. The United Nations warned on Monday that two million refugees in 10 African countries are facing "critical shortages of food assistance" because donors are failing to keep pace with the rapidly rising need.

The number of refugees in Africa has nearly doubled in the past six years, reaching nearly five million today. In some countries, refugees have had their food rations cut in half.