At least 22 others were injured after attackers raided and burned the village of Duk Padiet, governor Kuol Manyang added.
All wounded people have been evacuated to Juba, the capital, he said.
A series of attacks between ethnic groups in the region has displaced tens of thousands of people.
“We are expecting more to be injured because they ran to the villages last night,” Kuol Manyang said.
Officials told AFP news agency the killings were carried out by the Murle group on the Lou Nuer as revenge for an attack last month on the village of Pibor.
The cycle of violence has lasted months and killed hundreds of people. It began as cattle raids but has spiraled out of control.
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