According to the statistics by the United Nations, 270,000 people have fled their homes in south-western Syria since the military launched an assault on rebel-held areas two weeks.

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.

More than 130 civilians have been killed since hostilities escalated, according to the Syrian Observatory for Human Rights, a UK-based monitoring group.

Deraa and Quneitra had been relatively calm for almost a year because of a "de-escalation" agreement brokered by the US and Jordan, which support the opposition, and Russia, a staunch ally of the government.

But Mr Assad set his sights on regaining full control of the provinces after defeating rebels in the Eastern Ghouta region outside the capital Damascus in April.

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.

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.

UN High Commissioner for Human Rights Zeid Raad Al Hussein warned of a humanitarian catastrophe in Deraa and urged neighbouring countries to provide safe passage to those wishing to flee the violence.

Jordan has kept its border closed and said it will not take in any more refugees, while Israel has said it will not allow Syrians to cross into its territory.

After meeting UN officials on Monday, Jordan's Foreign Minister Ayman Safadi told reporters that it had been delivering tents and medical supplies to crossings with Syria, but that there had been issues getting it across the border to those in need.

Mr Safadi also said he would hold talks with his Russian counterpart in Moscow on Tuesday, which he hoped would "take us more steps forward to contain this crisis and prevent more destruction".

The Israeli military has also stepped up its humanitarian efforts.

On Friday, it took in six injured Syrians, including four young children, for emergency medical treatment. It also supplied tents, food, baby formula, medical equipment, clothing and shoes to camps in the Golan Heights demilitarised zone.