Ten relief workers traveling in South Sudan have been abducted by an armed group, the United Nations said today. The kidnapping underscores the risks to humanitarian aid providers in the war-torn African country.

A statement from the U.N. said the aid workers, all from South Sudan, disappeared Wednesday during a trip through the southern part of the country. The statement gave no details on the armed group that seized them or its demands.

This is the third time in six months that a group of aid workers has been held by militants in South Sudan, the world’s youngest country. At least 98 aid workers have been killed in South Sudan since it devolved into civil war in 2013, two years after it was founded. The majority of the aid workers killed since the conflict began are South Sudanese.

The U.N. statement called for the release of the workers “without condition so that their work can continue.”

“We are deeply concerned about the whereabouts of these humanitarian workers and are urgently seeking information about their well-being,” Alain Noudehou, the U.N. humanitarian coordinator for South Sudan, said in the statement. Aid organizations working in the country, he said, “are here to help the people of South Sudan and should not be targeted.”

The aid workers disappeared as their convoy was traveling from Yei to the town of Tore, about 50 miles northwest, in the restive Central Equatoria region.

The South Sudanese army, loyal to President Salva Kiir, has repeatedly clashed with rebel groups loyal to deposed Vice President Riek Machar. Both sides have been accused of targeting aid workers and committing widespread human rights abuses.

Because of the violence in the region, much of the Greater Equatoria area has become inaccessible to humanitarian groups.

Yei is controlled by an opposition rebel group loyal to Machar, but its governor, Frank Matata, said he did not believe the group was behind the kidnapping. Earlier this week, he said, there were skirmishes in the area between the rebels and a breakaway faction. He said it was not immediately known who is holding the aid workers or where they are.

“I do not believe it was us,” Matata said, “because I was not notified. They would have told me.”

Lam Paul Gabriel, a spokesman for the main rebel group in South Sudan, the SPLM-IO, said he did not know whether his group was behind the kidnapping, which he learned about Wednesday when a U.N. official called to report missing staff members.

“I cannot give a clear statement that it was us or it was not us,” he said.

The area of the kidnapping has no cellphone coverage. Lam said members of the IO force had been dispatched from the group’s nearest base, which is two days away on foot.

“If the aid workers were taken by us, we need to know why,” he said.

All of the missing are South Sudanese citizens working for the U.N. and other international organizations. One is from the U.N. Office for the Coordination of Humanitarian Affairs; two from UNICEF; one from the South Sudanese Development Organization, two from Across, a Christian aid group; three from Plan International; and one from Action Africa Help.

“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.