As America continues to debate whether or not to intervene in Syria, millions of citizens are fleeing the country. Jana Mason of the United Nations Refugee Agency stops by to discuss the conditions on the ground (The Washington Post)

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 are 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 ­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 — 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, most recently, the alleged use of 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.

Caught in the middle: The contrast in lives between those who stayed back and those who decided to leave Syria.

The real number of refugees is almost certainly higher than 2 million, because many do not register with authorities. More than half of them are children, the United Nations says. In addition, an estimated 4.25 million people are displaced within Syria.

Countries overburdened by the influx have begun to impose limits on the number of Syrians who come in. Egypt’s new military-backed rulers are revoking visas that were issued to more than 100,000 Syrians under a policy put in place by the government of Mohamed Morsi, who was ousted in July.

Jordan and Turkey, each hosting about half a million Syrians, are limiting 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 biggest number of Syrian refugees and has not imposed restrictions, although local news outlets have quoted unnamed officials as saying that there are plans to do so.

The United Nations says 718,104 Syrians have registered in Lebanon, but the government puts the refu­gee figure in excess of 1 million, meaning that one in five people living in Lebanon is a Syrian.

Iraq has recently seen one of the biggest surges of refugees, notably to the semiautonomous Kurdistan region, one of the last places allowing Syrians almost unrestricted access. There are 171,984 registered refugees in Iraq, the United Nations says.

Angelina Jolie, a special envoy for the Office of the U.N. High Commissioner for Refugees, said the countries absorbing the refugees urgently need massive international support if they are to continue providing sanctuary to Syrians. She warned the world against becoming “dangerously complacent” about the humanitarian disaster.

Timeline: Unrest in Syria

Two years after the first anti-government protests, conflict in Syria rages on. See the major events in the country's tumultuous uprising.

“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.”