Desperate South Sudan villagers, fleeing fighting across the country, are eating grass and roots to survive as humanitarian organisations start costly air drops of food to northern parts of the country.
UN High Commissioner for Refugees António Guterres said on Tuesday the international community had a moral responsibility to help.
"One family told me they were boiling poisonous roots for six or seven days to take out the poison in order to have something to eat," he said from the town of Nyal in Unity state.
"These people are in risk of starvation."
James Puot, 50, told the Associated Press news agency he walked for 20 days with his wife and 10 children to get to Nyal. He said they were eating water lilies, roots and grass while drinking water from the Nile River.
"We've been waiting a whole day for some food,'' he said. "We've had no water. We've spent days without water."
A massive Ilyushin plane has started dropping more than 30 tons of food in the northern states. Each drop provides at least 15 days of rations for 18,000 people, intended to help those stranded between pro-government and anti-government troops.
But the air drops, three times more expensive than road deliveries, are straining the ramped-up humanitarian response because only a third of the UN's requested $1.27bn has been raised for the crisis.
The UN's response has been hampered by some missteps. The reported discovery of weapons in a UN convoy on March 10 prompted the South Sudan army and government to crackdown on all UN vehicles carrying aid. The issue further strained already shaky relations between the government and the UN.
Several UN agency staff in Juba told AP the cost of road delivery was now 25 percent more expensive as UN contractors are refusing to leave the capital with goods bound for neighbouring states due to harassment by soldiers and checkpoints that charge up to $10,000 in "inspection fees'' for goods to pass.
Ertharin Cousin, UN World Food Programme's top official, acknowledged an "access problem" but said President Salva Kiir assured WFP conditions would improve.
"This is one of many complications we are working to resolve." she said.
Nearly seven million people are at risk of hunger, according to the Food and Agriculture Organisation.
The UN reports that in the 100 days since the South Sudan conflict started more than one million people have fled their homes and 3.7 million are now at high risk of food insecurity.