The regime has been pounding the southwestern province of Quneitra since Sunday in a bid to retake it from rebels, after winning back most of the neighbouring governorate of Daraa in less than a month.

The Syrian Observatory for Human Rights said air strikes killed 14 civilians in Ain al-Tina village on Quneitra's border with Daraa which had reportedly been taking shelter in a large building.

"They were all displaced from other areas. They included five children and three women," Observatory chief Rami Abdel Rahman said.

Two bodies were so charred they were unrecognisable.

It was not immediately clear whether the strikes were carried out by the regime or its Russian ally, the Britain-based monitor said.

The monitor determines who carried out strikes based on the type of aircraft and munitions used, locations and flight patterns.

Activists shared images of the strikes' reported aftermath, showing motionless children with greyish skin wrapped in blood-stained blankets.

Russian air raids killed one civilian near the village of Al-Aliya in Daraa province, the Observatory said.

"Since Tuesday morning, heavy Russian air strikes and barrel bombs dropped by the regime have been targeting an area straddling Quneitra and Daraa," Abdel Rahman said.

Under pressure from the increasing bombardment, rebels in at least five towns in Quneitra had agreed to a negotiated surrender to the government, Abdel Rahman said.

- 'Open the border' -

There was no immediate confirmation from state media on the new surrenders in Quneitra province.

Other areas along the border between the two governorates were holding out, particularly those held by Hayat Tahrir al-Sham, a group led by Syria's former Al-Qaeda affiliate.

HTS was not included in the broad ceasefire deal for Daraa, which was brokered earlier this month by Russia.

The regime has heavily bombed HTS-held territories since Sunday, leaving 48 jihadists and rebels dead as opposition attacks have killed at least 43 regime fighters, according to the Observatory.

Syrian President Bashar al-Assad has recaptured more than 90 percent of Daraa province since beginning a fierce bombing campaign on June 19.

But a western part of the province, as well as most of adjacent Quneitra, still escape his control.

Quneitra is a thin, crescent-shaped province wedged between Daraa to the east and the buffer zone with the Israeli-occupied Golan to the west.

Tens of thousands of displaced civilians remain displaced in the area near the buffer zone and are terrified of the encroaching hostilities.

But Israel -- like Jordan to Syria's south -- has kept the border closed to desperate displaced families fleeing the assault in recent weeks.

On Tuesday, some 200 Syrians approached the Israeli-controlled border fence in the Golan but were told to back away, an Israeli military spokesman said.

"The women were calling out, 'Open the border, save whoever is left of our children,'" said Bahaa Mahameed, a Syrian doctor.

"They started waving us away with their hands and telling us to get back for our own safety," he told AFP from Quneitra.

More than 350,000 people have been killed and millions displaced by Syria's war since it started in 2011 with a brutal crackdown on anti-government protests.