"These are groundless accusations. It is not clear what they are based on. No specific data has been given. We do not agree (with them)," said presidential spokesman Dmitry Peskov in response to a US briefing saying that Russia was responsible for the attacks.

The Britain-based Syrian Observatory for Human Rights has also blamed Russian warplanes for the strikes.

Bombardment of the enclave on the outskirts of Damascus on Tuesday killed 106 civilians, including 19 children, the Observatory said.

It was the second straight day that the civilian death toll topped 100, after 127 were killed Monday in Eastern Ghouta's bloodiest day in four years.

The strikes left an important hospital out of action, further limiting the little medical aid accessible to besieged civilians.

The rebel-held region is nominally included in a "de-escalation" deal meant to tamp down violence, but President Bashar al-Assad appears to be preparing troops for a ground assault to retake it.

The Russian military has fought a campaign for over two years in Syria, launched in September 2015 in support of Assad, helping to turn around the multi-front war.