Six aid workers have been killed in an ambush in

A humanitarian source working in the country told AFP that the victims, three Kenyans and three South Sudanese working for a local aid group, were travelling from the capital Juba to the eastern town of Pibor in a convoy when the attack took place on Saturday.

Their car was stopped by unknown assailants and "they were taken from the vehicle and shot and killed," the source said on condition of anonymity.

Three years of civil war in South Sudan have seen warring parties deny access to aid, attack humanitarian workers and loot relief supplies.

The conflict has displaced about 2.5 million people from their homes and fields and created a devastating humanitarian crisis, including a famine affecting 100,000 and threatening another one million.

"I am appalled and outraged by the heinous murder yesterday (Saturday) of six courageous humanitarians in South Sudan," Eugene Owusu of the UN Office for the Coordination of Humanitarian Affairs (OCHA) 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."

According to OCHA, it was the highest number of aid workers killed in a single incident since the conflict began in December 2013 in the world's youngest country.

79 aid workers killed

The ambush follows two other attacks on aid workers this month.

A health worker and a patient were killed in an attack on a humanitarian convoy responding to a cholera outbreak in the central town of Yirol on March 14.

During fighting in the northern area of Mayendit on March 10, local staff of an international NGO were detained by armed rebels and released four days later, said OCHA.

The agency said at least 79 aid workers have been killed in South Sudan since December 2013.

This year alone 12 have died and at least eight humanitarian convoys have been attacked.

Analysts and United Nations officials have accused the government of using starvation as a battlefield tactic.

A leaked report by UN investigators said last week it was no coincidence that areas afflicted by famine are opposition areas, home to mostly ethnic Nuer and controlled for the most part by rebels.

Michele Sison, the US deputy representative to the United Nations, told a Security Council meeting on Thursday that the government's obstacles to humanitarian work in the famine-struck areas "may amount to deliberate starvation tactics."

'Peace, not food aid'

South Sudan's leaders fought for decades for independence, but once they it was achieved in 2011, the fighting turned inward.

A long-standing power struggle between President Salva Kiir and his former deputy Riek Machar sparked fighting in Juba in December 2013 which quickly turned into a wider conflict between Kiir's Dinka supporters and Machar's Nuer community.

Since then the fighting has spread across the country and among ethnic groups jockeying for political and military advantage and to protect their communities.

UN chief Antonio Guterres on Thursday denounced "a refusal by the leadership to even acknowledge the crisis or to fulfil its responsibilities to end it".

Regional peace efforts have borne no fruit and the UN has been unable to push through an arms embargo or compel Kiir's government to accept the deployment of a regional protection force.

According to OCHA some 7,5 million people in South Sudan are in need of humanitarian assistance.

"Only a political plan to end South Sudan's national crisis, not food aid, can bring actual famine relief to South Sudanese," Alan Boswell, a conflict analyst and writer on South Sudan told AFP last week.