Six aid workers have been killed in an ambush in famine-hit South Sudan, the United Nations said on Sunday, without specifying what organisation they worked for.

The victims were attacked on Saturday as they travelled from the capital, Juba, to the eastern town of Pibor, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).

"I am appalled and outraged by the heinous murder of six courageous humanitarians in South Sudan," Eugene Owusu, the UN humanitarian coordinator for South Sudan, said in a statement.

"At a time when humanitarian needs have reached unprecedented levels, it is entirely unacceptable that those who are trying to help are being attacked and killed," Owusu added.

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According to OCHA, it was the highest number of humanitarian workers killed in one incident since the start of a civil war more than three years ago.

Saturday's ambush followed two other attacks on aid workers this month, shortly after South Sudan declared famine last month in two counties of Unity state, saying 100,000 people face starvation and another million are on the brink of famine.

After gaining independence from Sudan in 2011, the world's youngest nation descended into war in December 2013 after President Salva Kiir accused his rival and former deputy Riek Machar of plotting a coup.

Tens of thousands have since died and 3.5 million people have been displaced, while a surge in fighting since July has devastated food production in areas that had been stable for farmers.

At least 79 humanitarians have been killed in South Sudan since the conflict began, including at least 12 killed this year, OCHA said.

"These attacks against aid workers and aid assets are utterly reprehensible," Owusu said. "They not only put the lives of aid workers at risk, they also threaten the lives of thousands of South Sudanese who rely on our assistance for their survival."

Earlier this month, the government announced plans to charge a levy of $10,000 for each foreign aid worker, which along with the danger of abduction could hurt efforts to help the hungry.

On Thursday, UN Secretary-General Antonio Guterres accused South Sudan's government of ignoring the plight of tens of thousands of people suffering from famine, 7.5 million in need of humanitarian aid and thousands fleeing the fighting.

"There is a strong consensus that South Sudanese leaders need to do more to demonstrate their commitment to the wellbeing of the country's people, who are among the poorest in the world," Guterres said during a UN Security Council briefing.

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Joseph Moum Malok, South Sudan's deputy UN ambassador, said the government "takes issue with the accusation", adding that other parts of the country are affected by drought.

He also said the government "will spare no efforts to help address the situation and calls upon the international community to help address this urgent matter".

A confidential UN report, which was seen by the AFP and Reuters news agencies earlier this month, found that the famine was largely caused by Juba's military operations.

"The bulk of evidence suggests that the famine in Unity state has resulted from protracted conflict and, in particular, the cumulative toll of repeated military operations undertaken by the government in southern Unity beginning in 2014," the report said.

It also said the government is spending its oil revenue on weapons, even as the country descends into a famine.

The government rejected the report's allegations.