The attack was carried out near the town of Mangalla, north of the capital Juba, on Wednesday, UNICEF said in a statement.
"We have received reports that two trucks belonging to a UNICEF contractor were assaulted by armed men," the agency said, adding that an assistant driver was killed in the assault, Presstv Reported.
The UN agency said it was still trying to verify the name and nationality of the victim.
UNICEF also denounced "this senseless attack directed against unarmed civilians working to deliver humanitarian supplies to those in need."
Elario Paul Fataki, a local government official in the area where the incident occurred, said the attack took place around five kilometers from Mangalla.
South Sudan, the youngest country in Africa, has been gripped by a bloody civil war since December 2013, when incumbent President Salva Kiir accused his former deputy, Riek Machar, of plotting a coup.
The two sides have been involved in a cycle of retaliatory killings that have split the impoverished country along the ethnic lines. Tens of thousands have been killed and millions displaced in the conflict.
Frequent attacks on humanitarian convoys and personnel have been reported in South Sudan's conflict and both warring sides have been blamed.
At least 100 charity workers have been killed in the war.