Ten aid workers have gone missing in Yei River State, says the UN Office for the Coordination of Humanitarian Affairs.
Their convoy was en route to Tore from Yei town, early on Wednesday to conduct a humanitarian needs assessment.
OCHA said their whereabouts and well-being are unknown.
“We are deeply concerned about the whereabouts of these humanitarian workers and are urgently seeking information about their well-being,” said Alain Noudehou, the Humanitarian Coordinator for South Sudan.
The ten humanitarian staff, all nationals, include one from UNOCHA, two from UNICEF, one from the South Sudanese Development Organization, two from ACROSS, three from Plan International and one from Action Africa Help.
Mr Noudehou demanded their swift return.
“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 ten humanitarian staff have gone missing just a week after seven other aid workers were released by Dr Riek Machar forces in Yei River State.
The workers, from the South Sudan Health Association, were abducted in March while travelling in a 3-vehicle convoy from Kupera County to Lainya County.
The SPLA-IO had told Eye Radio that their intelligence unit detained the humanitarian workers because they were found to be agents spying for the government.
However, it later said only two were government agents but they were all released on directives of Dr Machar.
This is the second incident involving aid workers being held by armed groups in April alone, and the third in six months, according to OCHA.