Russia bears responsibility for recent reported chemical weapons attacks by the Syrian government, US Secretary of State Rex Tillerson said Tuesday.

The alleged chlorine gas attacks in January affected 20 civilians, many of them children, in the rebel-held Damascus suburb of Eastern Ghouta, where government-allied forces have starved and pummelled around 400,000 civilians in a three-plus year siege.

"Whoever conducted the attacks, Russia ultimately bears responsibility for the victims in East Ghouta and countless other Syrians targeted with chemical weapons, since Russia became involved in Syria," Mr Tillerson told reporters in Paris.

"There is simply no denying that Russia, by shielding its Syrian ally, has breached its commitments to the US as a framework guarantor."

The top US diplomat’s harsh rebuke will further strain relations between the two superpowers inside Syria. Moscow backs the Syrian government headed by President Bashar al-Assad and the US seeks his removal.