Russia has violated its duty to guarantee the destruction of Syria's chemical weapons and prevent the Assad government from using poison gas, the United States says.
Syria signed up to the international ban on chemical weapons in 2013, as part of a deal brokered by Moscow to avert US air strikes in retaliation for a nerve gas attack that killed hundreds of people, which Washington blamed on Damascus. In the years that followed, Syria's declared stockpile of banned poison gasses was destroyed by international monitors.
Washington said it has evidence that Syria has used chlorine gas in attacks in recent weeks. Rescue workers and medics on the ground have described residents choking on fumes after air strikes.
Unlike nerve gas, chlorine is legal for countries to possess for water purification and other civilian purposes, but using it as a weapon is banned.
Russian Foreign Minister Sergei Lavrov, addressing a UN-sponsored Conference on Disarmament, said Syria had eliminated its chemical weapons stockpile and placed its arsenal under international control. Washington was using bogus claims of chemical weapons use to attack the Syrian government, he said.
The world's chemical weapons watchdog in the Hague opened an investigation on Sunday into attacks in the besieged, rebel-held Syrian region of eastern Ghouta to determine whether banned munitions had been used, diplomatic sources told Reuters.
Robert Wood, US disarmament ambassador, told reporters in Geneva shortly before Lavrov addressed the forum that Syria was violating the chemical weapons convention it signed in 2013.
He said Moscow had "clearly been in violation" of its commitment to act as a "guarantor" of Syria's behaviour.
The US envoy said he had no information on any Russian complicity in the alleged use of chemical weapons, but added: "Russia is on the wrong side of history with regard to chemical weapons use in Syria."
But Mr Lavrov said that the United States was relying for its information on allegations by what he called "fully-discredited" Syrian rescue workers in rebel-held areas.