Sixteen children were among 20 civilians killed overnight when an airstrike on the town of Arbin levelled the school above the basement they were sheltering in, AFP reported Tuesday (March 20th).

The Syrian Observatory for Human Rights said the bombing raid hit Arbin, a key town in the dwindling opposition-held enclave of Ghouta that has been under attack by government troops for over a month.

"Three missiles from a single airstrike hit the school, where the underground level was being used as a shelter," said Observatory head Rami Abdel Rahman.

"Rescue workers are still searching for survivors," he said.

Monday night's raids were suspected to have been carried out by Russia, the Observatory said.

Moscow has said it is helping Syria's government "finish off" fighters in Ghouta but has denied carrying out airstrikes against civilians.

Since February 18th, Syrian troops and allied militia have captured more than 80% of Eastern Ghouta and have splintered the remaining territory into three sections, each held by a separate opposition group.

The pocket where Arbin lies is held by the Faylaq al-Rahman Islamist faction.

Syrian troops have made sweeping advances against them in recent days, opening a "corridor" for civilians to flee into government-controlled territory.

Other residents have opted to flee deeper into the shrinking opposition-held areas.

Meanwhile, the regime continued to pound Eastern Ghouta's main town of Douma even as a trickle of emergency medical evacuations was scheduled to continue Tuesday.

An AFP correspondent reported heavy bombardment through the night, adding that ambulances were struggling to reach the wounded so intense was the shelling.