Conflicts and drought have driven up the number of southern Sudanese in need of food aid from one million in 2009 to 4.3 million this year, the United Nations and Southern Sudan's government said Tuesday.
Southern Sudan has suffered from a drought, which also hit the wider region, while tribal conflicts and incursion by a Ugandan rebel group have displaced hundreds of thousands.
Southern Sudan's Agriculture and Forestry Minister Samson Kwaje said Jonglei State - which has seen much of the tribal violence - was the worst hit.
"Internal conflict and incursion from the Lord's Resistance Army together with drought have made almost half the population of the South short of food," he said.
Some 2,500 people were killed and 350,000 displaced in 2009 as a result of tribal clashes sparked by cattle raids, the UN said.
Tribal disputes have long been common in autonomous Southern Sudan but easy access to weapons left over from the civil war between the Muslim north and Christian and animist south has helped ramp up the body count.
Harvests have also been slashed by drought, and the WFP said it was pre-positioning 50,000 metric tonnes of food to feed people who may be cut during the rainy season, which is due to begin shortly.
"This spike in the number of hungry people in southern Sudan comes just ahead of the rainy season when roads become blocked and communities are cut off from food assistance," said Leo van der Velden, WFP Sudan Coordinator in the south.
However, WFP said that it was almost $500 million short of the funds necessary to feed 11 million people in north and Southern Sudan in 2010.