The South Sudanese nationals were working for an Italian aid agency, when they were ambushed by heavily armed men as they returned from delivering nutritional supplies to health facilities in Yirol area.
The UN said their vehicle was clearly identifiable and have called on authorities to immediately investigate the killings.
It comes amidst a sharp rise in killings and violence across South Sudan that has claimed the lives of more than two hundred people since mid-May.
Four aid workers have been killed in South Sudan this year alone, all in the past month.
The attacks have raised fears over possible suspension of humanitarian activities that could affect millions of people in need of food aid.
