The number of refugees fleeing Syria's civil war has topped two million - a nine-fold increase in 12 months, according to the United Nations.
"Syria has become the great tragedy of this century - a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history," said UNHCR head António Guterres.
The UNHCR - the UN's refugee agency - wants a massive increase in aid to stop Syria's neighbours buckling under the strain.
A year ago, the number of Syrians registered as refugees or awaiting registration stood at 230,671 people.
The massive rise comes as France bolstered calls for military action in Syria by saying it had evidence the regime was behind a "massive and co-ordinated" chemical attack in Damascus.
The US government has also said evidence points to deadly sarin gas being used to kill 1,429 people on August 21.
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. Some 515,000 have gone to Jordan, 460,000 are in Turkey, while Egypt has 110,000 and Iraq 168,000.
The UNHCR said the countries were struggling to provide food and shelter at refugee camps.
It wants the international community to act quickly to stop the system collapsing.
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 year civil war.
Actress Angelina Jolie, a UNHCR Special Envoy, said the international community had a "responsibility to do more" to help Syrian refugees.
"If the situation continues to deteriorate at this rate, the number of refugees will only grow," said Jolie. "Some neighbouring countries could be brought to the point of collapse."
Meanwhile, US President Barack Obama has been meeting political figures such as Senator John McCain as he tries to galvanise support for military strikes ahead of a debate in Congress.
But in Britain, another Parliamentary vote on the issue seems unlikely.
Deputy Prime Minister Nick Clegg said he could not foresee any circumstances in which the Government would take the vote back to Parliament.
France's intelligence report, which includes satellite imagery, claims chemical attacks were launched from government-controlled areas, a French government source told Reuters news agency.
It claims they killed "at least 281" people.
French MPs will debate the issue on Wednesday, but President Francois Hollande does not need parliamentary approval for military action lasting less than four months.
Syria's government called the allegations "illogical" and warned it would retaliate if France took military action.
The crisis is set to be top of the agenda among world leaders at the two-day G20 summit, which begins in St Petersburg on Thursday.