At least 270,000 people have fled their homes in south-western Syria since the military launched an assault on rebel-held areas two weeks ago, the UN says.
Many of those displaced by the fighting in Deraa and Quneitra provinces have headed towards the borders with Jordan and the Israeli-occupied Golan Heights.
But neither country has said it will allow an influx of refugees, sparking fears of a humanitarian “catastrophe”.
Government forces have been advancing with the help of Russian air strikes.
On Sunday, rebels in the major town of Bosra al-Sham reportedly agreed to lay down their arms and accept President Bashar al-Assad’s rule.
BBC reports that UN officials estimated only six days ago that up to 50,000 people had fled their homes in rebel-held towns and villages in response to the government’s intense air and artillery strikes. By Monday, the figure was more than five times higher.
“We were expecting the number of displaced in southern Syria to reach 200,000, but it has already exceeded 270,000 people in record time,” Mohammad Hawari, a spokesman for the UN Refugee Agency in Jordan, told AFP news agency.
Up to 70,000 of the displaced are reportedly gathered near the closed Nassib border crossing with Jordan, where many families are being forced to live in makeshift shelters or out in the open, with limited access to food or water.