A confidential report by both the United Nations and the global chemical weapons watchdog has alleged Syrian government forces carried out at least two toxic gas attacks and the Islamic State of Iraq and the Levant reportedly used sulfur mustard gas.
According to the joint report seen by the Reuters news agency, a year-long probe by the UN and Organisation for the Prohibition of Chemical Weapons (OPCW) - authorised by the UN Security Council - focused on nine attacks in seven areas of Syria, where a separate OPCW fact-finding investigation had already determined that chemical weapons were likely to have been used.
Eight of the attacks investigated involved the use of chlorine. The inquiry was unable to reach a conclusion in six cases, though it said that three of those cases warranted further investigation.
The 15-member Security Council is due to discuss the report next week. UN Secretary-General Ban Ki-moon said the report would be made public after that meeting.
The inquiry found there was sufficient information to conclude that Syrian government helicopters dropped devices that then released toxic substances in Talmenes on April 21, 2014 and Sarmin on March 16, 2015, both in Idlib governorate. Both cases involved the use of chlorine.
The Syrian mission to the United Nations did not immediately respond to a request for comment on the finding of the report.
It also determined there was sufficient information to conclude that ISIL were the "only entity with the ability, capability, motive and the means to use sulfur mustard gas in Marea on August 21, 2015".
Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington. The Security Council backed that deal with a resolution that said 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 authorisation of military force by the Security Council. The body would need to adopt another resolution to impose targeted sanctions - a travel ban and asset freeze - on people or entities linked to the attacks.
However, Russia - a close Syrian ally - and China have previously protected the Syrian government from council action by blocking several resolutions, including a bid to refer the situation in Syria to the International Criminal Court.
"The use of these weapons is abhorrent and we unequivocally condemn those who unleash them," British UN Ambassador Matthew Rycroft told the council on Monday. "This council must stand ready to demonstrate a robust response to this report."
Deputy US Ambassador to the United Nations Michele Sison told the council on Tuesday: "Those responsible for the use of chemical weapons in Syria must be held accountable for their actions."
In the case of the attack in Sarmin, the UN/OPCW inquiry found that the remnants of the device dropped "are consistent with the construction of a barrel bomb". Barrel bombs are steel drums full of shrapnel and explosives dropped from the air.
It said attacks in Kafr Zita in Hama governorate on April 18, 2014, Qmenas, in Idlib governorate on March 16, 2015, and Binnish in Idlib governorate on March 24, 2015, merit further investigation.
The inquiry did not recommend further investigation of the remaining three cases in Kafr Zita on April 11, 2014, and Al-Tamanah on April 29-30, 2014, and May 25-26, 2014.
The separate OPCW fact-finding investigation had found that chlorine was "systematically and repeatedly" used as a weapon during the Syrian conflict. Government and opposition forces have denied using chlorine.
Chlorine's use as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013. If inhaled, chlorine gas turns to hydrochloric acid in the lungs and can kill by burning lungs and drowning victims in the resulting body fluids.
"The UN Security Council should now ensure that those responsible for these attacks are brought to justice in a court of law. 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.