Three others were wounded, including a health officer working for the International Organization for Migration.
The aid workers had been treating a cholera outbreak in the town of Yirol, 200km north of the capital, Juba
The head of the IOM, William Lacy Swing, condemned the attack which he said happened in an area of the country in dire need of assistance.
Parts of South Sudan have been declared to be in famine after three years of civil war and a severe drought.
source:bbc