A United Nations investigation has established that Syrian regime leader Bashar al-Assad's forces carried out at least two chemical attacks and that DAESH used mustard gas as a weapon in the conflict that has ravaged Syria since 2011.
The year-long UN and Organization for the Prohibition of Chemical Weapons (OPCW) inquiry - unanimously authorised by the UN Security Council - focused on nine attacks in seven areas of Syria.
A separate OPCW fact-finding investigation had already determined that chemical weapons had likely been used in the attacks.
The results have set the stage for a UNSC showdown between the five veto-wielding powers, likely pitting Russia and China against the United States, Britain and France over whether sanctions should be imposed in the wake of the probe.
The 15-member Security Council is due to discuss the report next week and UN Secretary-General Ban Ki-moon said the report would be made public after that meeting.
Eight of the attacks investigated involved the use of chlorine, according to the two-page summary of the report.
Chlorine's use as a weapon is prohibited under the 1997 Chemical Weapons Convention. 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 panel was able to identify the perpetrators of three chemical attacks carried out in 2014 and 2015.
The inquiry was unable to reach a conclusion in six cases, though it said that three of those cases warranted further investigation.
The report from the Joint Investigative Mechanism (JIM) found that the Syrian regime dropped chemical weapons on two villages in northwestern Idlib province: Talmenes on April 21, 2014 and Sarmin on March 16, 2015.
In both instances, Syrian air force helicopters dropped "a device" on houses that was followed by the "release of a toxic substance," which in the case of Sarmin matched "the characteristics of chlorine."
The panel found that DAESH "was the only entity with the ability, capability, motive and the means to use sulphur mustard" in an attack on the town of Marea in northern Aleppo province on August 21, 2015.
The Assad regime has repeatedly denied that it has used chemical weapons in Syria, but the report said that in all three cases, it had "sufficient information to reach a conclusion on the actors involved."
The JIM was set up by the Security Council a year ago to investigate the use of chemical weapons and for the first time to determine who is responsible for the attacks.
Most of the nine cases investigated pointed to the alleged use of chlorine gas in barrel bombs dropped from helicopters.
Britain, France and the United States had long maintained that only the Syrian regime has helicopters, but Russia, an ally of Damascus, insisted that there was no concrete proof that Assad's forces carried out the attacks.
US calls for swift action
US Ambassador Samantha Power called for "strong and swift action" by the Security Council to follow up on the findings of the report.
"It is essential that the members of the Security Council come together to ensure consequences for those who have used chemical weapons in Syria," she said in a statement.
In Washington, National Security Council spokesman Ned Price said "it is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people."
"The United States will work with our international partners to seek accountability through appropriate diplomatic mechanisms, including through the United Nations Security Council," he added.
The report "states clearly that the Syrian regime and DAESH have perpetrated chemical attacks in Syria," French Deputy Ambassador Alexis Lamek told reporters.
"When it comes to proliferation, the use of chemical weapons, of such weapons of mass destruction, we cannot afford to be weak. The council will have to act."
The Security Council is due to discuss the report on Tuesday and could decide to impose sanctions on Syria or ask the International Criminal Court to take up the matter as a war crime.
But many diplomats say Russia would be unlikely to back such a move, despite the JIM's strong findings of chemical weapons use in the three cases.
The panel recommended further investigation of three other cases of suspected chemical weapons on the village of Zafr Zita, in Hama province, on April 28, 2014, and on two towns in Idlib: Qmenas on March 16, 2015 and Binnish on March 24, 2015.
The 24-member team said there was insufficient information to reach a conclusion in three other cases and recommended that there be no further investigation of those suspected attacks.
The findings prompted immediate calls for the perpetrators to face justice.
"The UN Security Council should now ensure that those responsible for these attacks are brought to justice in a court of law," said Louis Charbonneau, Human Rights Watch's UN director.
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 and China have previously protected the Syrian regime from council action by blocking several resolutions, including a bid to refer the situation in Syria to the International Criminal Court.
The Syrian regime had agreed to join the Chemical Weapons Convention after a chemical weapons attack near the Syrian capital killed scores of people in August 2013.
Doctors Without Borders said 355 people had died of “neurotoxic” symptoms in the attack.
The United States and its allies alleged that Assad's forces carried out the attack with sarin nerve gas, killing more than 1,400 people, including 400 children. Russia and Assad blamed opposition forces.