The Syrian Observatory for Human Rights blamed the bombing raids on regime ally Russia.

"Five civilians including two women were killed on Sunday in Russian strikes on the towns of Al-Herak, Al-Sura, and Alma," said Rami Abdel Rahman, the head of the Britain-based monitor.

The raids on Al-Herak hit near a hospital, damaging it and forcing its medical staff to shut it down at least temporarily, he said.

The three rebel-held towns are located in the southern province of Daraa, known widely as the cradle of Syria's seven-year uprising.

Daraa and the adjacent province of Quneitra are mostly still held by opposition forces, but army troops appear to be preparing a ground assault to retake them.

They have been escalating their air strikes, artillery fire, and use of deadly barrel bombs on rebel territory and on Saturday made their first gains on the ground.

Later that evening, Russia began bombing rebel towns in Daraa for the first time since brokering a truce there with the US and Jordan in mid-2017.

A total of 23 civilians have been killed in opposition areas since the escalation began on Tuesday, according to the Observatory.

Rebel forces are also firing rockets and mortars into regime territory.

On Sunday, a girl was killed and three people were wounded in rebel fire on government-held districts of the city of Suweida, Syria's state news agency SANA said.

After securing Damascus, Syrian President Bashar al-Assad is keen on recapturing the country's south, but key parts of it fall under a "de-escalation zone" agreed last year by the US, Jordan, and Russia.

Those same powers are in talks now to reach a negotiated settlement for the south that could head off a bloody regime assault.

Assad said his troops would have "no choice" but to take the south by force if those talks failed.

Although the offensive has not started yet in full force, his troops have already captured four villages in the south, the Observatory said.

Thirteen regime forces have been killed in the fighting, as well as 15 rebels, it added.

Many of those rebels have previously received backing from Jordan and the US, but Washington has warned them that they should not expect American help should the regime start a new assault.