Kampala - More than 1.5 million South Sudanese have become refugees and their humanitarian needs are overwhelming aid efforts during the country's civil war, according to the United Nations.South Sudan's civil war began in December 2013 and roughly 3.6 million people have fled their homes or become refugees, according to the UN. The country is Africa's largest refugee crisis and the third largest in the world, after Syria and Afghanistan.The humanitarian situation in South Sudan has "deteriorated dramatically," said Eugene Owusu, the UN aid chief in South Sudan, who described the country as troubled by the threat of famine and widespread sexual violence."We are facing unprecedented needs, in an unprecedented number of locations," Owusu said, adding that $1.6bn is needed to respond to the crisis.Roughly 7.5 million people are in need of assistance and protection, a majority of the country's estimated 12 million population, according to the UN. Around 4.6 million people are expected to receive food assistance in the first part of 2017, according to the World Food Program.South Sudan's government, and to a lesser extent rebel forces, have blocked UN peacekeepers and humanitarian assistance in parts of the country, despite repeatedly promising unfettered access, according to aid organisations.Recent fighting between government and rebel forces in the Wau Shilluk area of the Upper Nile region have caused humanitarian organisations to temporarily suspend their operations there.A December letter from South Sudan's National Security Service ordered aid organisations to "immediately pullout," without giving a reason, from Panyijar in the Unity region, where thousands of displaced civilians were receiving assistance. Some aid organisations say they have since been allowed back in the area.South Sudan's government spends roughly half of its national budget on defense spending. Since 2005, the US has sent roughly $11bn in aid to South Sudan.South Sudan's civil war has killed tens of thousands of people and a peace deal signed in August 2015 has failed to stop fighting.