GENEVA — A daily exodus of villagers fleeing armed conflict, hunger and sexual violence in South Sudan has pushed the number of refugees sheltering in Uganda to more than 1 million, the U.N. refugee agency said Thursday, urging international action to deal with what it called one of Africa’s biggest humanitarian crises. International relief agencies say that one-third of the South Sudanese population of 13 million people has now been displaced and that half of the population is suffering from severe hunger and is in need of food aid. “We’re looking at Africa’s biggest displacement crisis,” Adrian Edwards, a spokesman for the agency, said in an interview.