Backed by Russia, Syrian government forces have ramped up air strikes and ground attacks on rebel positions in the south, particularly its main province of Daraa.

"Our estimates are 45,000, maybe even higher," said Linda Tom, spokeswoman for the UN humanitarian coordination office (OCHA) in Syria.

The displacements have been happening increasingly fast in recent days as violence has escalated, Tom said.

"Over the last couple of days we saw a very large number of people who fled because of the ongoing hostilities, because of the shelling and fighting in that area," she told AFP.

The majority are fleeing from the eastern parts of Daraa province south towards the sealed border with Jordan, which has said it cannot absorb any more refugees.

"We haven't seen massive displacement in this scale in Daraa," Tom said.

The UN had previously warned that fighting was putting 750,000 people in rebel-held areas of the south in harm's way.

More than five million people have fled abroad since the Syrian conflict erupted in 2011. Another six million are displaced inside the country.