The two aid workers have been killed in separate attacks over the weekend in civil war-torn South Sudan, the United Nations said on Tuesday.A statement says armed men shot at a "clearly marked" aid vehicle near Bentiu town in Unity state, killing a local worker with the Hope Restoration organisation. And a local worker with the UNIDO organisation was killed near Leer town in Unity state.They are the first two aid workers to be killed this year in South Sudan, which is one of the world's most dangerous places for aid workers. At least 98 have been killed since the fighting began in December 2013, most of them local workers.The UN also says seven local workers with the SSUHA aid organisation were abducted March 25 by opposition forces while delivering health supplies in Morobo county in Central Equatoria. The UN is demanding their unconditional release.The opposition says the aid workers weren't abducted but were taken because they didn't have "proper clearance.""Our investigation says some of them are government agents," opposition spokesperson Lam Paul Gabriel told The Associated Press. He said they were trying to "clear the case" so the workers can be released.In November, President Salva Kiir ordered unimpeded access for aid organisations throughout the country but attacks have persisted.Rights groups have called on all sides in the conflict to protect civilians and aid workers."The brutal killing and abductions of aid workers are possible war crimes and should not be tolerated," Jehanne Henry, senior researcher for Human Rights Watch, told the AP.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.