6 million people now displaced.

"Syria has become the great tragedy of this century - a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history," said the head of the UN's refugee agency, António Guterres.

More than half of the refugees created by the war - some 52% - are under 17, said the UNHCR, and over one million children have now left their homeland to try to escape the bloodshed.

The situation is said to have reached a "critical stage", with more than 5,000 Syrians spilling over borders each day into neighbouring countries.

Lebanon has taken the most people, with 716,000; 515,000 have gone to Jordan; 460,000 to Turkey; while Egypt has 110,000 and Iraq 168,000.

A further 4.25 million people are displaced inside Syria, according to UN data - meaning six million people have been forced to leave their homes during the two and a half years.

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