"One has to ask where is Russia, where is Iran, which had pledged in Astana to guarantee a ceasefire also in Eastern Ghouta," said Steffen Seibert, spokesman for Chancellor Angela Merkel, referring to peace talks in the Kazakh capital.

"Without the support of these two allies, Assad's regime would not be where it is today, and undoubtedly, without this support, this regime would have to show more readiness to negotiate in the UN (peace) process," the German government spokesman added.

"We demand that the Assad regime immediately end the massacre in Ghouta and allow in humanitarian aid as well as medical evacuation. We also demand that the backers of the Assad regime use their strong influence to achieve this end," Seibert said.

The regime of Syrian President Bashar al-Assad stepped up its airstrikes this month on the rebel enclave east of Damascus, where close to 300 civilians have been killed since Sunday.

Warplanes continued to pound Eastern Ghouta towns on Wednesday, killing 24 civilians, according to the Syrian Observatory for Human Rights.