The US has urged "strong and swift action" after a UN investigation found that Syria's government used chemical weapons against its own people.
The year-long inquiry found the government used chlorine gas in attacks in Idlib province in 2014 and 2015.
Chlorine is an industrial chemical and was not part of the weapons stockpile that Syria agreed to give up in 2013.
However, use of it as a weapon would contravene conventions signed by President Bashar al-Assad.
The US ambassador to the UN, Samantha Power, said the use of chemical weapons was "barbaric" and called for "all states to support strong and swift action".
Ms Power accused the Syrian government of violating a September 2013 resolution which orders the UN Security Council (UNSC) to impose measures for "any use of chemical weapons by anyone in the Syrian Arab Republic".
The investigation was carried out by the Joint Investigative Mechanism (JIM) of the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW), an international chemical weapons watchdog.
It also found that so-called Islamic State (IS) had carried out an attack using the blister agent sulphur mustard.
The JIM did not draw firm conclusions on other reported chemical weapons attacks which it investigated.
The report will be discussed by the security council next week.
Ned Price, a spokesman for the US National Security Council, said it was now "impossible to deny" the Syrian regime used weapons in violation of international law.
"The United States will work with our international partners to seek accountability through appropriate diplomatic mechanisms," he said, adding that Russia and Iran should participate with UN member states.
French ambassador to the UN Alexis Lamek also said "the council will have to act".
"When it comes to proliferation, use of chemical weapons, such weapons of mass destruction, we cannot afford being weak," he added.
Chemical weapons were used on a large scale in the Ghouta area near Damascus in 2013, according to a UN report at the time.
The Syrian government, supported by Russia, denied claims by the US, UK and France that it was to blame for the attacks.