Famine has been declared in parts of South Sudan, where UN agencies warned on Monday that war and a collapsing economy have left 100,000 people facing starvation.

The UN Food and Agriculture Organisation (FAO), the International Children's Fund (Unicef) and the World Food Programme (WFP) also called for urgent action as almost 5 million people urgently needed food, agricultural and nutrition assistance, Xinhua news agency reported.

The agencies said famine was currently affecting parts of Unity State in the northern-central part of the country, adding that a further 1 million people are classified as being on the brink of famine.

"A formal famine declaration means people have already started dying of hunger. The situation is the worst hunger catastrophe since fighting erupted more than three years ago," they said in a joint statement.

FAO Representative in South Sudan Serge Tissot said the famine had become a tragic reality in parts of South Sudan.

"Our worst fears have been realised. Many families have exhausted every means they have to survive," Tissot said.

"The people are predominantly farmers and war has disrupted agriculture. They have 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," he added.

According to the Integrated Food Security Phase Classification (IPC) update released on Monday by the government, 4.9 million people — more than 40 per cent of South Sudan's population — are in need of urgent food, agriculture and nutrition assistance.

Jeremy Hopkins, Unicef Representative in South Sudan, said over 1 million children are estimated to be acutely malnourished across the country while over a quarter of a million children are severely malnourished.

The IPC report estimates that 14 of the 23 assessed counties have global acute malnutrition (GAM) at or above the emergency threshold of 15 per cent, with some areas as high as 42 per cent. 

The upsurge in violence since July 2016 has further devastated food production, including in previously stable areas.

Soaring inflation, up to 800 per cent year-on-year, and market failure have also hit areas that traditionally rely on markets to meet food needs. 

Urban populations are also struggling to cope with massive price rises on basic food items.