The U.N. World Food Programme says three of its staff members are missing in the state of Upper Nile and will suspend operations in some areas because of safety concerns.

The agency said in a statement Wednesday that it is "extremely worried" about the fate of the missing workers, who disappeared on April 1 while on a trip to distribute food.

The statement said the men were traveling in a convoy from Malakal, the state capital, to Melut when intercommunal fighting" erupted. The agency then lost contact with the workers.

South Sudan has been generally volatile since December 2013 as government forces try to put down a rebellion. Aid groups have reported increasing harassment of humanitarian workers.