"The bombing still hasn't stopped. There are three warplanes in the sky and two helicopters," Firas al-Doumi, a rescue worker inside Douma, told AFP on Saturday morning.

Douma is the last rebel-controlled town in Syria's Eastern Ghouta, a sprawling suburb of Damascus that was once the opposition's bastion on the edge of the capital.

Backed by Russia, Syrian troops have recaptured 95 percent of Ghouta through a fierce air and ground assault, as well as negotiated withdrawals.

In an apparent bid to pressure Jaish al-Islam -- the Islamist group that holds the town -- to withdraw, Syria's government on Friday resumed bombardment of the town after a more than week-long lull.

Air strikes and shelling on Friday left 40 civilians dead including eight children, according to the Syrian Observatory for Human Rights monitoring group.

The Britain-based Observatory said warplanes were hitting across Douma on Saturday, as regime artillery fire hit neighbouring agricultural fields.

Syrian troops matched the renewed bombardment on Friday with a ground operation in the orchards surrounding Douma.

"The regime is trying to tighten the noose around Douma from the west, east, and south," said Observatory head Rami Abdel Rahman.

The resumed assault comes after the apparent failure of negotiations between Jaish al-Islam and regime backer Russia over a rebel withdrawal from Douma.

Top Jaish al-Islam political figure Mohammad Alloush on Friday blamed international supporters of Syria's government for hamstringing the talks.

"The talks were going well," he said, but power struggles between the regime's allies had caused them to break down.

"Their only shared interests is the blood of civilians," he wrote on Twitter.