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."
The war has disrupted farming and left people with little choice but to scavenge for food to survive.
"People have been pushed to the brink, [they are] surviving on what they can find to eat in swamps," said Emma Jane Drew, Oxfam's humanitarian program manager in South Sudan.
"Vulnerable people, out of reach of life-saving assistance due to the conflict, are paying the ultimate price," she added in a statement.
Famine a 'man-made tragedy'
Drew said the famine was "a man-made tragedy" and called for an end to the fighting so aid could get through to those most in need.
George Fominyen, the UN food program spokesman in South Sudan's capital, Juba, said the problem had been building for years.
"It has not been sudden," he said. "Food insecurity, hunger, malnutrition has been getting steadily worse since the conflict started three years ago."
Fomiyen said humanitarian groups had found it extremely difficult to reach the hardest-hit areas.
"We have to talk to 10 to 15 people and ask if it's possible to send a team there," he said. "You cannot just access these places without prior agreement."
Fomiyen said the program's food supplies will run out unless it can secure "a substantial injection of funds" -- $205 million -- within the next six months.
"We are quite concerned that we do not have the resources," he said. "We could run out of food by the end of June. The needs are so huge; every time you are entering a new front, a new battle."
Africa's biggest refugee crisis
Last week the United Nations warned that 1.5 million people had fled South Sudan, crossing the border into Uganda to escape the fighting in their homeland, and creating Africa's biggest refugee crisis.
Nasir Abel Fernandes, senior emergency coordinator for the Office of the United Nations High Commissioner for Refugees, said Ugandan refugee camps are filling up with desperate South Sudanese citizens
"More attention needs to be paid to South Sudan," Fernandes said. "The leaders really should be asked to take responsibility and stop this killing of civilians."
South Sudan is the world's newest country. It gained its independence from Sudan in 2011, after years of civil war.
But after two years of relative peace, trouble broke out between President Salva Kiir's mainly Dinka army and the Nuer people of his former deputy Riek Machar.
Since July 2016 that fighting has intensified to draw in other ethnic groups and render the countryside, including the formerly agriculturally rich Equatoria region, a permanent war zone that has been producing refugees instead of food.