"There were 16 killed, two of them children, in intensive air raids carried out by the regime against several regions of Eastern Ghouta," the Syrian Observatory for Human Rights said.

"The casualty toll could rise because there are people stuck under the rubble and several wounded in a critical condition," Rami Abdel Rahman, the head of the Britain-based group, said.

The latest bloodshed came a day after strikes and shelling killed at least 31 civilians in the area, which has been besieged by government forces since 2013.

Several children were among Monday's dead.

The raids also came after a heated exchange between the United States and Russia at the United Nations over the reported use of chemical weapons by President Bashar al-Assad's regime.

Regime or allied forces have fired suspected chlorine-filled munitions on targets in Eastern Ghouta on several occasions in recent weeks, sparking US warnings of military action.

Russia, Assad's main backer, retorted that "no perpetrators have been identified" and accused the United States of orchestrating a "propaganda campaign" against the Syrian government.