The global chemical weapons watchdog said March 1 that chlorine was used against the rebel-held Syrian town of Douma last year, in a long-awaited final report on the attack that killed 43 people.
The report by the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW) was based on a visit by a fact-finding mission to the site of the attack.
The mission's mandate does not include laying blame, but Western powers led by the United States said the regime of President Bashar al-Assad was responsible for the attack and launched air strikes on Syrian military installations in response.
The OPCW said two cylinders likely containing chlorine smashed into a housing block in the town.
The report said there were "reasonable grounds that the use of a toxic chemical as a weapon has taken place on 7 April 2018. This toxic chemical contained reactive chlorine."
It said, however, that it found no evidence of the use of nerve agents in Douma, which had been previously alleged by some parties in the conflict.
The Russian Embassy in The Hague rejected the report, claiming the attack was "staged" by Syrian rescue volunteers known as the White Helmets.
Russia is Assad's main international backer.
The report will now go to the UN Security Council.