Ninety percent of the total number of refugees have left Syria with the past 12 months, the U.N. High Commission for Refugees said in a news release Tuesday.
"The war is now well into its third year and Syria is hemorrhaging women, children and men who cross borders often with little more than the clothes on their backs," the release said.
UNHCR head Antonio Guterres said Syria has become "a disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history."
Guterres said "the only solace is the humanity shown by the neighboring countries in welcoming and saving the lives of so many refugees."
The flow of refugees is putting "an overwhelming burden" on the infrastructures, economies and societies of neighboring countries taking in refugees from Syria, the UNHCR said.
"They urgently need massive international support to help deal with the crisis," the news release said.
UNHCR Special Envoy Angelina Jolie said the world is at risk of becoming "dangerously complacent about the Syrian humanitarian disaster."
"The tide of human suffering unleashed by the conflict has catastrophic implications," Jolie said. "If the situation continues to deteriorate at this rate, the number of refugees will only grow, and some neighboring countries could be brought to the point of collapse."
Jolie said the world is "tragically disunited" on how to resolve the conflict in Syria but "there should be no disagreement over the need to alleviate human suffering, and no doubt of the world's responsibility to do more."
"We have to support the millions of innocent people ripped from their homes, and increase the ability of neighboring countries to cope with the influx," she said.
An average of about 5,000 people flee Syria each day, the UNHCR said.