In a statement from the State Department, Washington threw its weight behind a UN call for a ceasefire in rebel-held Eastern Ghouta and demanded that Moscow rein in its ally.

The British-based Syrian Observatory for Human Rights estimates that four days of government air strikes on Eastern Ghouta, which is held by rebels opposed to Bashar al-Assad's regime, have killed more than 200 civilians.

"These attacks must end now," spokeswoman Heather Nauert said.

"The United States is extremely concerned about escalating violence in Idlib, the Damascus suburbs of Eastern Ghouta and other areas of Syria threatened by ongoing regime and Russian airstrikes.

"We are yet again appalled by the recent reports of the Assad regime's use of chemical weapons and the escalation of bombings that has resulted in dozens of civilian deaths in the last 48 hours."

US officials believe that Syria has used chlorine as an illegal chemical weapon in recent strikes in Ghouta and elsewhere.

"The United States supports the United Nations calling for a month-long cessation of violence to allow the delivery of humanitarian supplies and the urgent medical evacuation of over 700 civilians in Eastern Ghouta," Nauert said.

"Russia must use its influence with Damascus to ensure the Syrian regime immediately allows the UN to provide vital assistance to this extremely vulnerable population."