The use of chlorine as a weapon is banned under a 1997 convention
A United Nations inquiry has found Syrian government forces responsible for another toxic gas attack, this time in Syria's Idlib province in March 2015.
It is the third gas attack so far that the inquiry by the UN and the Organisation for the Prohibition of Chemical Weapons, the global chemical weapons watchdog, has blamed on President Bashar al Assad's forces.
In a confidential report, submitted to the UN Security Council, the inquiry said military helicopters were used to drop barrel bombs on Idlib, which released toxic chlorine gas.
The use of chlorine as a weapon is banned under the 1997 Chemical Weapons Convention, which Syria joined in 2013.
If inhaled, chlorine gas can kill by burning the victim's lungs and drowning them in the resulting body fluids.
The finding sets the stage for a showdown in the UN Security Council between Russia and western council members over how to respond.
The report said the names of the individuals who had command and control of the helicopter squadrons at the time could not be confirmed, but said those responsible "must be held accountable".
In an earlier report by the inquiry, President Assad's forces were blamed for chlorine attacks in Talmenes on April 21st, 2014 and in Sarmin on March 16th, 2015.
It also said Islamic State militants had used sulfur mustard gas.
Syria agreed to destroy its chemical weapons under a deal brokered by Moscow and Washington in 2013.
The deal was backed by the UN Security Council, which passed a resolution saying that in the event of non-compliance, "including unauthorised transfer of chemical weapons, or any use of chemical weapons by anyone" in Syria, it would impose measures under Chapter 7 of the UN Charter.
Chapter 7 deals with sanctions and the authorisation of military force by the Security Council.
However, the Council would need to pass another resolution to impose any sanctions on people or entities found to be linked to the attacks.