Unidentified attackers killed two aid workers who were returning from a refugee camp in a western Ethiopia region that shares a border with South Sudan, their aid group and a U.N. organization said.

The attack happened on Thursday in Gambella region, which shares a porous frontier with South Sudan, when the staff of U.S.-based Action Against Hunger were returning from a child nutrition centre at Nguenyyiel Refugee Camp.

“Team members were in transit from our 24-hour pediatric Nutrition Stabilization Center in Nguenyyiel Refugee Camp when they were ambushed by armed individuals,” Action Against Hunger said in a statement on Thursday.

“Two employees were killed at the scene. Action Against Hunger has suspended full operations in Gambella, but are maintaining the provision of life-saving assistance.”

Nguenyyiel camp hosts just over 74,000 refugees from South Sudan, according to data from the U.N. refugee agency UNHCR.

The refugees are part of 2.3 million who have fled South Sudan since 2013 when it descended into a civil war, just two years after gaining independence from Sudan, sparked by a feud between President Salva Kiir and his former deputy Riek Machar.

The conflict has killed 400,000 people, and also forced another 1.8 million out of their homes within South Sudan’s boundaries and crippled production of oil, the country’s main revenue earner.

“I am confident that Ethiopian authorities will arrest and prosecute those responsible for the attack,” Steven Were, U.N. Resident and Humanitarian Coordinator in Ethiopia, said in a statement on Thursday.

There was no immediate comment from Ethiopia authorities on the incident.