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The Organisation for the Prohibition of Chemical Weapons said last Friday that there were "reasonable grounds" to believe toxic chemicals containing "reactive chlorine" had been used in the attack, which witnesses said killed 43 people.

It said two cylinders likely containing the chemical had smashed into a housing block in Douma, which was held by rebels at the time. 

The team had no mandate to assign responsibility for the attack, but Western powers led by the United States blamed the government of Syrian President Bashar al-Assad, unleashing airstrikes on regime military installations in response.

A spokesman for Syria's foreign ministry said Thursday the government rejected the OPCW's findings in their entirety.

The official "called on member states of the OPCW to denounce such false reports, which lack credibility", accusing the OPCW of being "biased and unobjective", state news agency SANA said, in the first official response to the OPCW report.

The report was based on a visit to Douma by OPCW inspectors.

The team took more than 100 samples from seven sites in the town, to which the regime had denied them access for several weeks.

The OPCW said it reached its conclusions based on "witnesses' testimonies, environmental and biomedical samples analysis results, toxicological and ballistic analyses from experts".

Friday's report also denied the Syrian regime's claims that the gas came from a rebel chemical weapons facility and storehouse in the area.

"From the analysis of the information gathered during the on-site visits to the warehouse and facility suspected of producing chemical weapons, there was no indication of either facility being involved in their manufacture," it said.

The foreign ministry spokesman quoted by SANA said investigators were falsifying information.

"This is evidenced by their denial that armed terrorist groups posses toxic chemicals even though they found such chemicals" in warehouses owned by rebels, SANA quoted him as saying. 

The OPCW has investigated multiple chemical attacks during the eight-year Syrian civil war, and has previously confirmed the use of "chlorine, sulphur mustard, and sarin as chemical weapons" in other incidents.

The organisation previously had no mandate to assign responsibility for attacks, but has since been given powers to investigate responsibility for all chemical attacks in Syria back to 2014.

A report last month by the Global Public Policy Institute concluded that around 98 percent of the hundreds of chemical attacks in Syria since the start of the 2011 war were carried out by Bashar al-Assad's regime.

Thousands are thought to have been killed in the Syrian regime's chemical attacks against opposition areas. Most deadly has been the use of sarin, used in eastern Ghouta in 2013 and the Idlib village of Khan Sheikhoun in 2017.

The aim - as with the regime's use of barrel bombs - appeared aimed at creating terror in civilian areas rather than achieve a military victory.

Around 560,000 Syrians have been killed in the country's war, with the Syrian refugee crisis one of the largest displacements of people in modern times.