South Sudan faces a serious risk of famine by the end of this year and 30,000 people are already classified as being in a food security catastrophe, the Integrated Food Security Phase Classification (IPC) has said.

Hunger in the world's newest state has grown steadily worse in the nearly two years since a political crisis led to fighting that reopened ethnic fault lines between President Salva Kiir's Dinka people and ethnic Nuer forces loyal to former Vice President Riek Machar.

The two have signed a series of peace deals but fighting rages on.

The IPC, whose members include the UN Food and Agriculture Organization (FAO) and the World Food Programme (WFP), said famine had not been officially declared because it was hard to get data from conflict zones.

"There is a great concern that famine may exist in the coming months but it may not be possible to validate it at that time due to lack of evidence as the result of limited access to the affected areas and populations," it said.

Humanitarian groups have been forced to pull out of parts of oil-rich Unity State, one of the worst-hit areas, and they say displaced families are surviving on just one meal a day. In extreme cases, people fleeing violence survive by eating water lilies.