JUBA, South Sudan (AP) — The World Food Program said Friday it is "horrified" to learn that three of its South Sudan workers were killed this week in violence in the western town of Wau, as the country's civil war continues under warnings of possible genocide.

A statement from the U.N. agency said the three men had been contracted to work as porters and appear to have been killed Monday on their way to a WFP warehouse.

"Two died of machete wounds and the third was shot," the statement said. It did not give any details about the attackers.

WFP Country Director Joyce Luma said she is "outraged," and she called on South Sudanese authorities to hold the attackers accountable.

Wau residents on Monday described to The Associated Press targeted ethnic killings by government soldiers that spread panic through the town. The U.N. mission in South Sudan said its workers saw the bodies of 16 civilians in a hospital and said another 10 people were injured.

Residents said soldiers singled out civilians of the Fertit and Luo ethnic groups in retaliation for a rebel attack on government forces. Residents said the dead included children who had been on their way to school.

A South Sudan government spokesman, Michael Makuei, could not confirm Monday's violence.

The fighting came to Wau after government soldiers were killed in an ambush on Sunday south of the town, the U.N. mission's statement said. Troops, tanks, and equipment were seen moving in Wau late last week.

U.N. officials have repeatedly warned that South Sudan is at risk of genocide. Its three-year civil war has grown more intense in recent weeks. At least 50,000 people have died since the conflict began in December 2013, and the country has created Africa's largest migrant crisis with 1.8 million refugees.