The number of Syrian refugees now tops 2 million compared to about 200,000 a year ago according to the United Nations High Commission for Refugees.
The human wave is destabilizing nations like Lebanon, Iraq and Jordan while overwhelming countries like Turkey.
"The war is now well into its third year and Syria is haemorrhaging women, children and men who cross borders often with little more than the clothes on their backs," the agency said in a statement.
António Guterres, the UN high commissioner for refugees, said Syria had become "a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history".
"The only solace is the humanity shown by the neighbouring countries in welcoming and saving the lives of so many refugees," he added.
Tiny, volatile Lebanon has received the biggest number of refugees, both in per capita and absolute terms. UNHCR puts it at 720,000, but some estimates say as many as a million refugees have entered this country of 4m, and the government is under pressure to control the increase.
Many in Lebanon see the influx of Palestinian refugees after the creation of the state of Israel in 1948 as one of the triggers for the country's 15-year civil war, and, wary of the example, the Lebanese government has so far not allowed the creation of refugee camps.
This means Syrian refugees are dispersed among the population, often in the poorest areas, where locals complain they drive up the cost of housing. Thousands are living in temporary accommodation, in tents and on construction sites.
With the influx showing no sign of stopping, relations with host communities are becoming strained. There are reports of Syrian refugees being put under curfew in some towns in the mountains. In Beirut, streets once bustling with well-heeled shoppers are now crowded with destitute Syrians begging for money.
Jordan has meanwhile absorbed more than half a million Syrian refugees and hosts a camp, Zaatari, which has grown so large it is now the country's fourth-largest city. The government says the border remains open, but there are concerns that the numbers now managing to enter are artificially low, with reports that refugees have been stranded on the Syrian side of the border.
In Turkey, officials say that with close to 500,000 refugees in special camps and scattered throughout the country, they simply cannot absorb more people.
With humanitarian agencies recieving only about half of what they need to deal with the crisis, host countries have to make up the slack. They can't very well ignore the problem or they would have food riots involving hundreds of thousands of people.
The fighting is only becoming more ferocious. That means that the flood of refugees will increase and the need for concerted world action becomes more urgent.