The UN investigators also found evidence that the Islamic State of Iraq and the Levant (ISIL) group used mustard gas at least once in the conflict.
However, the UN Security Council failed during a closed-door session on Tuesday to agree on any action, with Russia questioning the evidence from the independent commission.
The international team of inspectors has determined that both the Syrian government and ISIL were responsible for chemical attacks carried out in 2014 and 2015.
Vitaly Churkin, the Russian ambassador, said it was too early to consider implementing a September 2013 Security Council resolution authorising sanctions that can be militarily enforced for any use of chemical weapons in Syria.
Russia has been a close ally of the Syrian government and President Bashar al-Assad since the crisis began there in 2011.
“Clearly there is a smoking gun. We know that chlorine most likely has been used – that was already the finding of the fact finding mission before – but there are no fingerprints on the gun,” Churkin said following the closed-door session.
“There is nobody to sanction in the report which has been issued,” he said. “It contains no names, it contains no specifics … If we are to be professional we need to question all the conclusions.”
Churkin said, however, that he was pleased the report had confirmed the use of chemical weapons by ISIL, also known as ISIS.
For his part, Bashar Jaafari, Syria’s UN ambassador, also dismissed the report’s findings as biased.
“The conclusions contained in the report were totally based on statements made by witnesses presented by the terrorist armed groups,” he said.
“Therefore, these conclusions lack any physical evidence.”
Heading into the meeting, Samantha Power, the US ambassador, called the report “a landmark” and said she expected a Security Council resolution “soon”.
“It is the first official independent confirmation of what many of us … have presented substantial evidence of for a long time, and that is a pattern of chemical weapons use by the Syrian regime,” she said.
“It is incumbent on the council to act swiftly to show … we were serious about there being meaningful accountability.”
In September 2013, Syria accepted a Russian proposal to relinquish its chemical-weapons stockpile and join the Chemical Weapons Convention.
That averted a US military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta.
Russia has blocked sanctions and other council action against Assad’s government.
However, it did support the establishment of the Joint Investigative Mechanism, charged with determining who was responsible for the attacks and paving the way for possible punishment.
The inspectors investigated nine cases in seven towns and determined the Syrian government was responsible for two attacks involving chlorine gas and the ISIL, which is already under UN sanctions, for one attack involving mustard gas.
They said three more attacks pointed towards government involvement but were not conclusive, and described three others as inconclusive.
Al Jazeera’s Mike Hanna, reporting from the UN headquarters in New York on Tuesday, said the mandate of the Joint Investigative Mechanism ends on September 23 and its work is not yet complete.
Source: Al Jazeera and agencies