Overall, more than 1.7 million people have registered as refugees since the uprising against the rule of President Bashar al-Assad began in March 2011, with many more believed to be unregistered.
Although Syrian refugees have fled to numerous countries, the vast majority have ended up in Lebanon, Jordan and Turkey.
Lebanon - the smallest of the three, with a population of four million - has more than 700,000 registered refugees. Jordan and Turkey are hosts to more than 960,000 others.