NAIROBI, Kenya (AP) — A U.N. official says that six aid workers were ambushed and killed in South Sudan, the deadliest attack on humanitarian workers since the East African nation's civil war began in 2013.

Eugene Owusu, the top U.N. humanitarian official in South Sudan, said in a statement Sunday that the attack took place on the road from Juba, the capital, to Pibor, where there have been recent reports of fighting. He did not identify the nationalities of the aid workers or their organization.

The ambush is the latest of several attacks on aid workers in South Sudan, which is one of the largest humanitarian crises in the world. At least 12 aid workers have been killed so far this year and 79 have been killed since civil war began in South Sudan.