Ten aid workers have gone missing in South Sudan in unclear circumstances just days after another group of humanitarians was abducted by gunmen, the United Nations Humanitarian Coordinator for the civil war-torn country, said Thursday. Alain Noudehou said in a statement that three UN staffers and seven aid workers, all of them South Sudanese, went missing early Wednesday when their convoy driving from Yei town to Tore in Central Equatoria disappeared. The aid workers are with South Sudanese Development Organization, ACROSS, Plan International and Action Africa Help. Noudehou condemned the latest attack against colleagues. This is the third time aid workers have been held by armed groups in the last six months alone, the statement said. Seven local aid workers seized by opposition forces earlier this month in the same area were later freed. Two other local aid workers were killed in a separate incident this month in Unity state. "We are deeply concerned about the whereabouts of these humanitarian workers and are urgently seeking information about their well-being," Noudehou said. South Sudan is one of the worlds most dangerous places for humanitarians. At least 98 have been killed since the civil war began in December 2013, most of them local workers. Neither the government nor the opposition claimed responsibility for the latest disappearance. The opposition leadership "is doing everything to get in contact with the commanders on the ground to find the truth," spokesman Lam Paul Gabriel told The Associated Press, pointing out the presence of many armed groups in the area. Army spokesman Lul Ruai Koang said the army was unaware of the incident.