The exodus of refugees accelerated in the second half of 2012, Wilkes said. A year ago, the United Nations had registered fewer than 20,000 refugees from Syria, which has a population of more than 22 million. Six months ago, the number rose to 120,000. By December, the number had climbed to 471,000. And now, nearly 585,000.
Jordan, Turkey and Lebanon are sheltering the most Syrians, with more than 150,000 each.
"We have in Jordan day shifts and night shifts" to register new refugees, Wilkes said. "In Lebanon, we have double shifts."
U.S. humanitarian aid
U.S. President Barack Obama has approved $155 million in humanitarian aid for people in Syria and refugees fleeing the violence.
That brings the aid total to $365 million, making the United States the largest single donor of humanitarian assistance to Syria.
The U.S. government has imposed sanctions against Syria, worked to isolate the regime, and backed the opposition but has refrained from intervening militarily.