Russia is partly to blame for a suspected chemical weapons attack in Syria, US Secretary of State Rex Tillerson has said.

"Russia ultimately bears responsibility for the victims in eastern Ghouta and countless other Syrians targeted with chemical weapons since Russia became involved in Syria," Mr Tillerson said after a conference on chemical weapons in Paris.

More than 20 civilians – most of whom were children – were hit by the attack on a rebel enclave, the Secretary of State said. He added that the incident raised "serious concerns" that Syrian President Bashar al-Assad was continuing to use chemical weapons against his own people.

Mr Tillerson said Russia had violated its agreement with the US to remove of chemical weapons from Syria, and had helped the country break an international treaty barring their use. Russia has long supported Mr Assad, even after a similar, suspected chemical weapons attack last year.

The Assad regime has denied using chlorine or other chemical weapons during Syria’s seven-year civil war.

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Monday's attack in eastern Ghouta – a rebel-held area suburb near Damascus – came as the government's bombing campaign in the area escalated, according to monitoring groups. The Syrian Observatory for Human Rights said at least 13 people were suffocated by what appeared to be chlorine gas.

US President Donald Trump ordered an air strike on Syria last April, after a suspected chemical weapons attack killed more than 100 people. The President blamed Mr Assad for the chemical attack at the time.

Vladimir Putin meets Syrian president Bashar Assad

Now, the US is increasing pressure on Russia to reign in its ally.

"Let’s be clear: Russia’s unwillingness or inability to restrain the Assad regime is costing innocent Syrian lives," State Department spokesperson Heather Nauert tweeted Monday. "We’ve been firm in our determination to hold parties accountable for the use of chemical weapons, which have killed far too many Syrians."

At Tuesday's meeting, Mr Tillerson urged Russia to stop vetoing UN Security Council resolutions that would hold governments that use chemical weapons accountable. If it could not do that, Mr Tillerson said, it should abstain.

The US diplomat spoke at the first meeting of a new organisation created to target governments that continue to use chemical weapons. The group of diplomats from 29 countries plans to publish information about chemical attacks and eventually sanction the perpetrators. The group is expected to put pressure on both Syria and Russia.

"Those who carry out chemical weapons attacks need to be made aware that we know who they are, and we will go after them," French Foreign Minister Jean-Yves Le Drian said.