Khartoum - UN agencies warned on Monday that an acute funding shortfall is hampering their work in assisting thousands of South Sudanese refugees who have fled to neighbouring Sudan.South Sudan became independent from Sudan in 2011, but two years later it descended into a brutal civil war that has killed tens of thousands of civilians.More than 50 000 South Sudanese have fled into Sudan since January to escape the violence and food shortages across the border.The UN High Commissioner for Refugees, the UN Children's Fund and the World Food Programme said they were facing a shortfall of more than $400m which was affecting their work."Our resources are being stretched at a time when needs are quickly growing," the UNHCR's Sudan representative, Mohamed Adar, said in a joint statement with UNICEF and the WFP."Further shortfalls in funding will hamper our ability to continue providing assistance for the existing South Sudanese refugees in Sudan, while also responding to the emergency needs of new arrivals." The UNHCR said its humanitarian requirements for 2016 were only 18% funded, "leaving over $128m in unmet needs".UNICEF and WFP said limited resources were hampering refugees' access to even basic needs."UNICEF is gravely concerned it may have to cut back on crucial lifesaving water, sanitation, nutrition, health and protection assistance" to more than 100 000 children from South Sudan, UNICEF said, adding it was facing a shortfall of $105m.The WFP, for its part, said it was facing a 12-month funding shortfall of $181m.In total, 678 000 South Sudanese refugees are currently being hosted in neighbouring countries with 221 000 in Sudan, the UN agencies said.Thousands more expected More refugees are expected to arrive ahead of the rainy season in South Sudan, the United Nations Office for the Co-ordination of Humanitarian Affairs (OCHA) said in a separate statement."A total of 93 000 South Sudanese refugees are anticipated in East Darfur by the end of June, with an additional 7 000 in South Darfur," it said.Until recently, the Sudanese government did not give them the status of refugees, instead according them many of the same rights and benefits as Sudanese citizens.But Khartoum ended that policy last month and said South Sudanese should be classified as "foreigners" as punishment for Juba's alleged support for rebels battling Sudanese troops in the border region.One of the world's least developed nations, South Sudan erupted into conflict in December 2013, pitting President Salva Kiir against his former deputy Riek Machar.The conflict has been marked by gross human rights abuses, including ethnic massacres and widespread rape.At least 50000 people have been killed, 2.4 million have been forced from their homes and 2.8 million need emergency food to survive.Under an August peace deal, Machar had been due to return to Juba on April 18 to forge a unity government with Kiir.But to the dismay of international donors, he had yet to return on Monday and fighting was still continuing in some areas.