More than 2 million Syrians have fled their country’s relentless civil war, the United Nations said Tuesday, an exodus that has doubled over the past six months and evolved into the worst refugee crisis in the world. An average of 5,000 people is crossing Syria’s borders each day. With violence in the country continuing to escalate and spread, the United Nations predicts that Syria’s refugee population could reach 3 million by the end of the year. Syria has become “the great tragedy of this century,” said Antonio Guterres, the U.N. high commissioner for refugees. He called the levels of suffering and displacement among Syrians “unparalleled in recent history.” Almost all of those who fled are being sheltered in neighboring countries.