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At least 23 civilians, including four children were killed and dozens more injured after a series of airstrikes hit a school and a hospital in eastern Damascus, a monitoring group reported Thursday.

The rebel-held town of Deir al-Asafir was struck by a heavy bombardment on Wednesday, the said, with the death toll likely to rise.

Among the dead were four children and a civil defence volunteer, the monitoring group said.

The attack was regarded as the "largest ceasefire violation in Eastern Ghouta," Rami Abdel Rahman, head of the Observatory said.

The fragile ceasefire, which was brokered by the US and Russia, had been intact across the area since 27 February.

More than 2,700 families live in Deir al-Asafir, the monitoring group confirmed.

The Syrian government forces have been trying to encircle the town for weeks, it added.

Syrian president Bashar al-Assad's government claims al-Qaeda's affiliate, al-Nusra Front, operates in the eastern suburbs of the Syrian capital.

But the militant group was excluded from the ceasefire which came into effect over a month ago.

The Syrian National Coalition [SNC] opposition group denounced the attack and called it a "massacre."

"This crime is the latest in a series of actions that aim to consecrate ongoing violations of the cease-fire," the SNC said in a statement.

There was no immediate reaction from the Syrian government, which rarely comments on security issues.

The ceasefire has been hailed as a success by the UN, which said the lull in fighting has allowed the pace of humanitarian aid deliveries to besieged areas to pick up.

More than 270,000 people have been killed since Syria's conflict erupted in March 2011.

Agencies contributed to this report.