Six aid workers were ambushed and killed at the weekend in South Sudan.
The attack took place Saturday on the road from the capital Juba to Pibor which has seen recent fighting.
The aid workers are yet to be identified.
“These attacks against aid workers and aid assets are utterly reprehensible,” said Mr Eugene Owusu, UN’s humanitarian chief in South Sudan. “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.”
The United Nations has described it as the deadliest attack against aid workers on record in the country.
Aid workers responding to both famine and cholera in South Sudan have been coming under attack from armed forces.
At least 12 have been killed this year alone. 79 others have died since the conflict began in 2013.
Ethnic violence has dominated the crisis in the world youngest nation South Sudan which gained its independence in 2011. The civil war began as a dispute between the Dinka President Salva Kiir, and former Vice-President Riek Machar who is Nuer.