Syrian refugees could double in number to more than a million by June, the UN has warned.
Those fleeing the fighting need urgent aid, said the UN's humanitarian agency, as it on Wednesday launched an appeal for $1.5bn to help those both inside and outside Syria.
"The violence in Syria is raging across the country," Radhouane Nouicer, regional humanitarian co-ordinator for Syria, told reporters in Geneva.
"There are really no more safe areas where people can flee," he added. "The magnitude of this humanitarian crisis is undisputable."
The appeal was to fund activities over the next six months, Nouicer said.
"The number of people in need of assistance inside Syria has quadrupled from one million in March 2012 to four million in December," said a statement from the UN's Office for the Co-ordination of Humanitarian Affairs.
But while an estimated two million people have now fled their homes and are still inside Syria - in desperate need of food, shelter, water and emergency medical services - the numbers of those reaching neighbouring countries is growing rapidly.
By June, the number of refugees registered in Jordan, Iraq, Lebanon, Turkey and Egypt was expected to have reached around one million, nearly double the 523,631 registered as of December 17, said the UN High Commission for Refugees (UNHCR).
"Since July, Syrian refugees have fled the conflict for neighbouring countries at a rate of 2,000 to 3,000 a day," the agency said.
"We are constantly shocked by the horrific stories refugees tell us," UNHCR regional co-ordinator Panos Moumtzis said in a statement "Their lives are in turmoil. They have lost their homes and family members. By the time they reach the borders, they are exhausted, traumatised and with little or no resources to rely on."
|Mohamed Vall reports from Lebanon's Bekaa Valley
on the plight of Palestinians fleeing to Lebanon
Many Palestinians in Syria - already refugees following dispossession and displacement from the 1948 and 1967 wars in their homeland - have fled to Lebanon in the recent week following fighting in Damascus.
Some of them have sought shelter in Al Jalil refugee camp in the Bekaa Valley.
"This is a small and crowded camp," Al Jazeera's Mohamed Vall reported from the camp. "Some 550 families have been received here - and this is only part of the 2,000 people Lebanese authorities say have crossed into Lebanon since clashes began in Yarmouk refugee camp in Damascus. There are tremendous problems here - no shelter, no food, and the cold."
The UN's appeal is to be split into two, with $519.6m requested for aid within Syria and $1bn to help those spread across the neighbouring five countries. The request comprises "the largest short-term humanitarian appeal ever", the UN said.
The agency actually requires far more than the $1.5bn requested, "but we are being realistic", Nouicer said.
For all of 2012, the UN had requested $835m to help Syrians inside and outside the country, although it received only $525m.