Thousands of civilians have fled the the southern opposition stronghold that borders Jordan and the Israeli-occupied Golan Heights as the Syrian regime intensified its offensive to retake the area.

A picture taken on June 21, 2018 shows people fleeing as smoke plumes rise from regime bombardment in Al Hirak in the eastern Daraa province countryside in southern Syria. Thousands of civilians have fled
A picture taken on June 21, 2018 shows people fleeing as smoke plumes rise from regime bombardment in Al Hirak in the eastern Daraa province countryside in southern Syria. Thousands of civilians have fled "intensified" regime bombardments of rebel-held areas in the southern Syrian province of Daraa. (AFP)

Thousands of civilians have fled "intensified" regime bombardments of rebel-held areas in southern Syria in the past three days, a monitor said Thursday, as fears mount of a full-blown assault.

President Bashar al Assad has set his sights on retaking rebel-controlled parts of southern Syria, whether through negotiations or a military operation, and has been amassing troops there in recent weeks.

So far, no deal has been struck to avert fighting over the southern opposition stronghold that borders Jordan and the Israeli-occupied Golan Heights.

On Thursday, a Britain-based war monitor said the regime had ramped up its bombardment of Daraa province, leading thousands to flee for their lives.

"More than 12,000 civilians have fled their homes in the last three days after regime forces intensified their shelling and air strikes on eastern" Daraa, the Britain-based Syrian Observatory for Human Rights monitor said.

The civilians fleeing areas including Nahta, Al Herak and Basr al Harir were "heading to nearby villages under rebel control not affected by the bombardment near the Jordanian border" to the south, Observatory head Rami Abdel Rahman said.

Opposition fighters control around two-thirds of Daraa, which borders Jordan, but the regime holds a sliver of territory in the centre of the province.

Daesh also holds a pocket of ground in the province.

Entire villages

The areas in eastern Daraa bombarded in recent days lie on a strip of land flanked by regime-held territory to the east and west.

A media activist in Basr al Harir told AFP thousands had escaped their homes in areas near his village. 

"Entire villages have fled," Mohammad Ibrahim said.

The UN humanitarian coordination office (OCHA) reported that 2,500 people had fled one of these areas, Nahta, as of Wednesday.

OCHA said the United Nations was "concerned about reports of an escalation of violence in Daraa... which is endangering civilians and causing hundreds of families to become displaced".

Daraa is largely considered to be the birthplace of the Syrian uprising in 2011 that eventually spiralled into civil war after a brutal crackdown, and the regime retaking it would be a symbolic victory.

Regime media confirms bombardment

State news agency SANA, using its customary term for rebels, said the army was shelling positions of "terrorists" in Al Herak and Basr al Harir on Thursday, and had killed a number of them.

In the neighbouring province of Sweida to the east, two civilians were killed on Thursday in rebel bombardment of the provincial capital of the same name, SANA said.

After a string of military victories against rebels earlier this year near Damascus, the regime has turned its attention to ousting rebels from areas of southern Syria.

These include parts of Daraa and the neighbouring province of Quneitra to the west, which is largely controlled by rebels and borders the Israeli-occupied Golan Heights.

De-escalation zones

Daraa, Quneitra and Sweida are included in a "de-escalation zone" agreed by the United States, Russia and neighbour Jordan last year.

Last week, the US State Department warned Damascus against any military action in the de-escalation zone.

In an interview with Iran's Al-Alam television channel also last week, Syria's Assad said contacts were ongoing between Russia, the United States and Israel over the southern front.

"We are giving the political talks a chance, but if they fail, there will be no choice but liberation by force," he said.

The regime has retaken large parts of Syria from the opposition since Russia intervened militarily on its side in 2015.

Earlier this year, Assad scored a major victory by retaking the former rebel bastion of Eastern Ghouta outside Damascus, after a blistering assault and a string of so-called evacuation deals.

That regime blitz displaced tens of thousands of civilians.

Syria's war has killed more than 350,000 people and displaced millions since it started in 2011 with the repression of anti-regime protests.