The Syrian government has carried out at least two attacks involving chlorine since 2014, while the Islamic state extremist group has used mustard gas, according to a confidential UN report seen by dpa Wednesday.
The findings show that the Syrian government has used chemical weapons despite a 2013 UN Security Council resolution requiring Syria to get rid of its chemical weapons stockpile and end such attacks.
The report by the joint investigative mechanism of the Organisation for the Prohibition of Chemical Weapons (OPCW) and the UN found that Syrian government helicopters dropped chlorine on the towns of Talmenes on April 21, 2014, and Sarmin on March 16, 2015, which are both in Idlib Governorate.
The report, which had been commissioned by the Security Council to identify perpetrators of chemical attacks in Syria, also found that the Islamic State fired artillery shells filled with mustard gas in Marea in Aleppo Governorate on August 21, 2015.
The investigators were unable to identify which side carried out chemical weapons attacks in six other cases.
"Lack of access to locations under investigation due to the dire security situation on the ground affected the manner in which the mechanism was able to conduct its investigation," the report said.
The use chemical weapons is prohibited under the Chemical Weapons Convention, to which Syria - along with 191 other countries - is a state party.
The US condemned the "horrific and continuous use of chemical weapons by Syria" and called on the Security Council to ensure that there would be consequences, according to a statement by Samantha Power, US ambassador to the UN.
"When anyone – from any government or from any terrorist group – so flagrantly violates the global ban on chemical weapons use without consequences, it sends the signal that impunity reigns and it gravely weakens the counter-proliferation regime from which all of us benefit," Power said.
Peter Wilson, deputy British ambassador to the UN, said his country would "be looking through this report very carefully to ensure that those accountable are held to account."
Human Rights Watch called the use of chemical weapons in Syria "sickening" and urged the council to bring perpetrators to justice.
"Without accountability, the cycle of abuses in Syria by all parties to the conflict – whether by chemical or conventional weapons – will continue unabated," said Louis Charbonneau, UN director at Human Rights Watch.