Half the population of Syria is expected to be in need of aid by the end of the year as millions of people are forced to flee their homes due to the civil war, the United Nations said Friday as it launched its biggest ever humanitarian appeal. In a statement, the U.N. High Commissioner for Refugees (UNHCR) agency said some $5 billion in aid would need to be spent during 2013 as the conflict is placing "an unbearable strain" on Syria's population. "Over 1.6 million Syrian refugees are now hosted across five countries. By the end of the year, it is estimated that half of the population of Syria will be in need of aid. This includes an anticipated 3.45 million Syrian refugees and 6.8 million Syrians inside the country, many of whom will be displaced from their homes," the UNHCR said. The U.N. refugee agency said that so far it had received $1 billion and was seeking a further $4 billion in "the largest humanitarian appeal in history." "Syria as a civilization is unraveling with as many as half of its citizens needing urgent help as a result of this savage conflict," said High Commissioner for Refugees Ant?nio Guterres.