Guterres urged all sides to uphold the basic principles of humanitarian law, including the protection of civilians.

"The secretary-general is deeply alarmed by the escalating situation in Eastern Ghouta and its devastating impact on civilians," said UN spokesman Stephane Dujarric.

Syrian and Russian air strikes battered rebel-held Eastern Ghouta, killing at least 106 civilians including 19 children on Tuesday, the Syrian Observatory for Human Rights said.

On Monday, 127 Syrians were killed.

"Nearly 400,000 people in Eastern Ghouta have been subjected to airstrikes, shelling and bombardment," said Dujarric.

Besieged by Syrian government forces, residents of Eastern Ghouta "are living under extreme conditions, including malnutrition," he added.

Guterres recalled that Eastern Ghouta has been designated as a de-escalation zone by Russia, Iran and Turkey and he reminded all parties "of their commitments in this regard."

Negotiations meanwhile were continuing at the Security Council on a draft resolution demanding a 30-day ceasefire to allow for urgent deliveries of humanitarian aid and medical evacuations.