At least 10 aid workers have gone missing in South Sudan's Yei River State, official said.
The UN relief coordinator, Mr Alain Noudehou, said the humanitarian community was shocked by the suspected kidnapping of the aid workers.
The 10 were on a mission to provide humanitarian aid to the residents of Yei.
They were reportedly kidnapped on Wednesday outside Yei town by unknown armed men.
"We are deeply concerned about the whereabouts of the humanitarian aid workers and are urgently seeking information about their well-being.
"Our colleagues must be released without conditions so that their work can continue," Mr Noudehou said.
RELATED CONTENT: South Sudan’s Kiir vows to crush rebels
Those missing include one staff from Unicef, one from the South Sudan Developmental Organisation (SSDO), three from Plan International and one from Action Africa Help (AAH).
Mr Noudehou condemned the kidnapping of the aid workers and urged the parties to the conflict to observe international humanitarian law.
"I firmly condemn the latest attack against colleagues engaged in emergency humanitarian assistance in central Equatoria and call on all parties to the conflict in South Sudan to ensure a safe and secure environment, conducive to the delivery of assistance," he said.
The kidnap is the second affecting aid workers in Yei River State in the recent times.