Top UN aid chiefs have begged do**rs to increase efforts to stave off looming famine in South Sudan, where a third of the population are in crisis after months of war. Nearly a million children aged under five face acute malnutrition, the world Food Programme (WFP) and UN children's agency UNICEF said in a joint statement released late Friday, after their top directors visited the impoverished nation. The UN Security Council said Friday the food crisis is **w the worst in the world, as it called on countries who had pledged 618 million dollars in aid to make good on their promises.