UN aid chiefs have begged donors to increase efforts to stave off a looming famine in South Sudan, where a third of the population are in crisis after months of war.

The world should not wait for a famine to be announced while children here are dying each and every day. Anthony Lake, UNICEF

Nearly a million children aged under five face acute malnutrition, the World Food Programme (WFP) and UN children's agency UNICEF said in a joint statement released late on Friday, after their top directors visited the nation.

"They fear the world is allowing a repeat of what occurred in Somalia and the Horn of Africa just three years ago; when early warnings of extreme hunger and escalating malnutrition went largely unheeded until official famine levels were announced," the statement read.

Without swift action, 50,000 children could die from malnutrition this year, they added.

"The world should not wait for a famine to be announced while children here are dying each and every day," UNICEF director Anthony Lake told the AFP news agency.

The UN Office for the Coordination of Humanitarian Affairs said aid agencies in the northern town of Leer were treating more than 1,000 cases of child malnutrition every month. Before violence broke out in December it was about 40 per month.

Thousands killed

The UN Security Council said on Friday that the food crisis is now the worst in the world, as it called on countries who had pledged $618 million in aid to make good on their promises.

Thousands have been killed and over 1.5 million people have fled more than seven months of fighting between government troops, mutinous soldiers and militia forces divided by tribe.

"If we are to rapidly expand our operations and save more lives, then we need more resources, and the international community has to act now," WFP chief Ertharin Cousin said.

A third of the population, nearly four million people, face "dangerous levels" of hunger, the UN said.

Fighting broke out in December, sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.

Top UN aid chief John Ging has called it a "man made problem, the result of a political disagreement between two powerful individuals."