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The U.N. World Food Program says it is “outraged” by an attack on a convoy carrying food aid in South Sudan that left one crew member missing and three with gunshot wounds.

A statement says the attack occurred Monday as the WFP convoy of boats was carrying food aid to Melut and Malakal in the north to assist hundreds of thousands of people displaced by recent flooding.

The missing crew member is presumed dead, the WFP statement says. It does not give details about the attackers. Country director Matthew Hollingworth called on South Sudanese authorities to hold those responsible for “this unspeakable violence.”

South Sudan remains one of the world’s most dangerous places for humanitarian workers and one of the most precarious nations. Two years after the end of a civil war that killed nearly 400,000 people, roughly half the population is hungry.

The U.N. has warned that South Sudan is one of a handful of countries facing potential famine.

The U.N. and other aid organizations have repeatedly called on South Sudanese authorities to make it safer and easier to move around the country.

Deadly intercommunal violence continues, and a peace deal signed two years ago faces delays in implementation.