Story highlights UN declares famine in South Sudan's Unity state

More than 1 million children across the country are suffering from acute malnutrition, UNICEF warns

(CNN) Famine in South Sudan has left 100,000 people on the verge of starvation and almost 5 million people, more than 40% of the country's population, in need of urgent help, aid agencies say.

People are already dying of hunger, and another 1 million people are on the brink of famine, UN agencies said.

Years of civil war, a refugee crisis and a collapsing economy have taken their toll on South Sudan since it gained its independence in 2011.

Now the UN World Food Programme and nongovernmental organizations are sounding the alarm, warning that more than a million children are suffering from acute malnutrition.

"Our worst fears have been realized," said Serge Tissot, of the Food and Agriculture Organization of the United Nations. "Many families have exhausted every means they have to survive."

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