JUBA (AFP) - South Sudan's "self-serving elite" of leaders and rebels are responsible for a looming "man-made famine", the departing United Nations chief for the country said Tuesday, on the eve of the war-torn nation's third birthday.

"Thousands and thousands have been killed," said Ms Hilde Johnson of the UN mission in South Sudan, lashing out at both the government and rebels, warning that one of world's least developed nations has "been set back decades".

Leaders were sick with "the cancer of corruption" with the country's billions of dollars worth of oil "a curse rather than a blessing," she said in a stinging criticism.

More than 1.5 million people have been forced to flee almost seven months of war, as a power struggle between President Salva Kiir and his former deputy Riek Machar spiralled into brutal ethnic conflict.

Aid agencies have warned that without massive funding, famine zones will be declared within weeks.

"We are at risk of seeing the worst famine in the country's history, and it is not because the rains did not come," Ms Johnson said in an uncharacteristically strong speech, made in the airport as she left the country for the last time.

"It is because of a man-made disaster, it is because of a man-made conflict, and if it comes, it will be a man-made famine."

The speech is likely to sour relations further between South Sudan and the UN, whose peacekeepers have come under attack during the conflict.

She placed the blame entirely leaders of the now bitterly divided ruling party, the Sudan People's Liberation Movement (SPLM), including both rebels and government, who had ruled "by the gun and not by the law".

"The leadership, across all factions of the SPLM, whether they are inside or out of government, released from detention or in the bush, are responsible for this," she said, adding that leaders could have stopped the violence that broke out on December 15, and had been warned it could lead to such devastating killings.

Ms Johnson, a former Norwegian development minister who helped mediate the 2005 peace deal that ended the two-decades long civil war that paved the way for South Sudan's independence, said the violence was the worst she had ever seen.

"Never before had you, or any of us, seen such killings and atrocities happen here, committed by South Sudanese against South Sudanese," she said.

"Never before had we seen the cities of Bor, Malakal and Bentiu virtually destroyed by fighting, and for the latter two, changing hands 12 times."

Peace talks in luxury hotels in the Ethiopian capital Addis Ababa have made little progress and last month they halted indefinitely, with both sides refusing to attend the discussions.

"If there are further delays, and the blame games go on, whether from those wanting to remain in office or those wanting to get back in, we can draw only one conclusion; that this is only about a scramble for power," Ms Johnson added.