Twelve peacekeepers and civilians working for the UN were killed on Tuesday in an ambush by unidentified assailants near Gumuruk in Pibor County of Jonglei state.
At least nine additional peacekeepers and civilians were injured in the attack and some remain unaccounted for – UNMISS, the UN Mission in South Sudan, said.
UNMISS Chief Hilde Johnson condemned the killings of five blue helmets and seven civilians.
She sent her deepest condolences to the families of all the peacekeepers, colleagues and civilians killed in the attack by unidentified gunmen.
Ms Johnson added that the attack will not deter UNMISS and its peacekeepers from working to protect vulnerable communities in the area.
This was the biggest loss of UN lives in South Sudan. In December, four Russian were killed in the same restive area when their aircraft was downed by the army.