BEIRUT — More than 2 million Syrians have fled their country's relentless civil war in an exodus that has doubled over the past six months and evolved into the worst refugee crisis in the world, the United Nations said Tuesday.
An average of 5,000 Syrians are crossing the borders each day. With violence inside the country continuing to escalate and spread, the United Nations predicts that Syria's refugee population could reach 3 million by the end of 2013.
Syria has already become "the great tragedy of this century," said António Guterres, the U.N. high commissioner for refugees. He called the levels of suffering and displacement among Syrians "unparalleled in recent history."
The vast majority of the refugees — 1.8 million — have left in the past year, as what began as a peaceful uprising in a country of 22.5 million people mushroomed into a full-blown war involving airstrikes, ballistic missiles and, allegedly, chemical weapons.
Almost all of those who fled are being sheltered in neighboring countries. Those nations, themselves fragile, are at risk of being destabilized by the vast and sudden influx of Syrians, who typically arrive with little money and few belongings.
The actual number of refugees is almost certainly higher than 2 million because many refugees don't register with authorities. More than half of those who have left are children, the United Nations says. In addition, an estimated 4.25 million people are displaced within Syria.
Countries already overburdened by the influx have begun to impose limits on more Syrians entering their borders. Egypt's new military-backed rulers are revoking visas issued to more than 100,000 Syrians under a policy put in place earlier by the Muslim Brotherhood.
Both Jordan and Turkey, each hosting about half a million Syrians, limit the numbers of refugees they will admit each day.
Lebanon, one of the region's smallest and most volatile nations, is giving sanctuary to the largest number of Syrian refugees and has not imposed restrictions yet.
The United Nations says 718,104 Syrians have registered in Lebanon, but the government puts the figure in excess of 1 million, meaning that one in five people currently living in Lebanon are Syrian.
Iraq has seen one of the biggest recent surges in the number of Syrians arriving, notably to the semi-autonomous northern province of Kurdistan, one of the last places in the region to allow Syrians almost unrestricted access. There are currently 171,984 registered refugees in Iraq, the United Nations says.
U.N. special envoy Angelina Jolie said the countries absorbing the refugees urgently need massive international support if they are to continue to provide sanctuary to Syrians. She warned the world against becoming "dangerously complacent" about the humanitarian disaster.
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"The tide of human suffering unleashed by the conflict has catastrophic implications," Jolie said. "If the situation continues to deteriorate at this rate, . . . some neighboring countries could be brought to the point of collapse."