A convoy of 10 South Sudanese aid workers has disappeared in the south of the country, the United Nations has announced.

The aid workers went missing on Wednesday morning shortly after departing from the southern city of Yei for Tore, some 80km away, the UN's Office for Coordination of Humanitarian Affairs (UNOCHA) said in a statement on Thursday.

The workers' wellbeing remains unknown, the statement added.

Among those missing are a UNOCHA member of staff and two from the UN children's agency, UNICEF.

Other members of the convoy included aid staff from Plan International, Action Africa Help, the South Sudanese Development Organisation and ACROSS.

'Deeply concerning'

Aid workers have frequently been targeted by armed forces operating in South Sudan since an ethnically-charged civil war erupted in December 2013 when troops loyal to President Salva Kiir and then-Vice President Riek Machar clashed.

A peace deal brokered between Machar and Kiir in August 2015 led to the creation of a unity government the following April.

The administration, however, broke down after only three months when violence erupted again between government forces and Machar's Sudan People's Liberation Movement-in-Opposition (SPLM-IO) fighters.

Alain Noudehou, the UN's humanitarian coordinator for South Sudan, said on Thursday he was urgently seeking information on the case.

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

"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."

Noudehou also called on "all parties to the conflict" in South Sudan to provide a safe environment within which aid can be delivered.

Aid workers at risk

Lem Paul Gabriel, a deputy spokesman for the SPLM-IO, said on Thursday the group's forces were investigating the incident, Reuters news agency reported.

"I am not denying or accepting that we are the ones responsible for this but we are investigating because that is our territory, but we should not rule out the presence of other armed forces," Gabriel said.

"So until we get that report from the ground commander, we will not be able to confirm."

Wednesday's incident marks the second occasion on which aid workers have been held by armed groups in April, and the third in six months, according to the UNOCHA.

Since its outset, the conflict has resulted in the deaths of tens of thousands of people - including at least 98 aid workers - and led to more than four million people being displaced from their homes, according to the UN.

The war - described by Filippo Grandi, the UN high commissioner for refugees, as having a human cost of "epic proportions" - has created Africa's biggest refugee crisis since the Genocide against the Tutsi in Rwanda in 1994.