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

A statement from the United Nations said the aid workers, all from South Sudan, disappeared on 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 aid workers have 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 United Nations statement called for the release of the workers “without condition so that their work can continue.”