The United Nations is seeking $1.3 billion in humanitarian aid for South Sudan, where two in ten of the population have been driven from their homes during two years of conflict.
More than 10,000 people have been killed and 2.3 million displaced since the country's civil war broke out in December 2013, when soldiers loyal to President Salva Kiir first clashed with troops who backed his deputy, Riek Machar.
Eugene Owusu, the U.N. Humanitarian Coordinator for South Sudan, said $1.3 billion would be the "bare minimum" needed to support 5.1 million people in the country facing life-threatening circumstances. "The challenge we face is unprecedented," he said.
The U.N. said more than 680,000 children below the age of five are believed to be acutely malnourished.
Much of the fighting has been along ethnic lines between Kiir's Dinka community and Machar's Nuer people.
Progress on a peace deal signed last year has been slow, with both sides accusing the other of violating the agreement and dragging their heels over plans to form a government of national unity.
The war has also devastated South Sudan's economy, slashing the oil production that funds most public spending.