South Sudanese women carry water in a UN camp in Juba, capital of South Sudan, Dec. 22, 2013. (Xinhua/Lu Rui)

JUBA, April 26 (Xinhua) -- The UN humanitarian agency confirmed on Thursday that 10 aid workers have gone missing outside South Sudan's Yei town in Central Equatoria region.

The UN Office for the Coordination of Humanitarian Affairs (UNOCHA) said the team departed Yei on Wednesday for a humanitarian needs assessment in the town of Tore, but lost contact with their offices and they still remain unaccounted for.

Alain Noudehou, UN Humanitarian Coordinator for South Sudan, said the whereabouts and well-being of the aid workers remain unknown and called for their immediate release.

"We are deeply concerned about the whereabouts of these humanitarian workers and are urgently seeking information about their well-being," Noudehou said in a statement issued in Juba.

"These individuals, UN and NGO staff, are here to help the people of South Sudan and should not be targeted. Our colleagues must be released without condition so that their work can continue," the relief chief added.

The ten humanitarian staff, all South Sudanese, includes one from UNOCHA, two from UNICEF and one from the South Sudanese Development Organization (SSDO). Three were from aid organization Plan International, two from ACROSS and one from Action Africa Help (AAH).

UNOCHA said this is the second incident involving aid workers being held by armed groups in April alone, and the third in six months.

The UN has described South Sudan as one of the deadliest countries for the delivery of humanitarian aid. At least 90 aid workers have been killed and dozens captured by armed groups since the east African country slid into violence in December 2013.

"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," Noudehou said.