The nerve agent sarin and toxic chemical chlorine were "very likely" used as weapons in two attacks in central Syria in late March 2017, said a global chemical weapons watchdog.
The Organisation for the Prohibition of Chemical Weapons said Wednesday that its fact-finding mission found "sarin was very likely used as a chemical weapon in the south of Ltamenah" in Hama province on March 24 and that chlorine was very likely used a day later at and near Ltamenah Hospital.
The OPCW's fact-finding team is not mandated to apportion blame for chemical attacks.
The organisation says its findings were based on witness testimony and analysis of samples.
A Syrian activist group documenting war crimes in April released a list of suspected chemical attacks since 2012, saying it had verified 861 videos from 193 sources covering some 212 attacks in Syria. Most of the material was collected from social media.
Earlier in the year, a report linked Syria's largest sarin nerve agent attack in August 2013 - which left hundreds dead - to the Syrian regime chemical stockpile.
A suspicious chemical attack on Douma in April this year left more than 49 dead, according to medical groups and rescuers, with other estimates reaching over 100, prompting punitive strikes by the US, UK and France.
Graphic images and videos emerged on social media following Douma's alleged gas attack, showing children struggling to breathe and entire families who had succumbed to the attack on the floors of underground shelters.
Russia said it dispatched experts to Douma who had not found any evidence of chemical weapons use. Later, the Russian Foreign Minister Sergei Lavrov claimed that Moscow had "irrefutable" evidence the attack was staged with the help of a foreign secret service.
For its part, France has cited "overwhelming testimony" and extensive analysis by French intelligence services and laboratories that pointed to the Assad regime's involvement.
Both Syria and Russia have denied using chemical weapons and have blamed the rebels on using it on themselves to whip up international condemnation, or claimed no attack took place at all.
Damascus joined the OPCW in 2014 and agreed to destroy its 1,300-tonne stockpile of industrial munition, under Russia's supervision, following a US-Russian deal.
Despite agreeing to the deal, inspectors have found evidence of an ongoing chemical weapons programme in the country, including systematic use of chlorine barrel bombs and sarin.
Around 500,000 people have died and millions made homeless in seven years of fighting in Syria, which was sparked when regime forces brutally put down peaceful protests in 2011.