More than four million Syrians - a sixth of the population - have fled abroad to escape the conflict in their country, the United Nations has said.
A surge in the number of people crossing into Turkey has increased the total by one million in just 10 months.
More than seven million others have been displaced inside Syria since the start of the uprising in March 2011.
The head of the UN's refugee agency, Antonio Guterres, called it the "worst humanitarian crisis of our generation".
The refugee crisis is the largest resulting from a single conflict in nearly a quarter of a century.