Syria blamed for chemical weapons attack in 2015
Syrian government forces carried out a third chemical weapons attack last year, a confidential report to the UN Security Council has found.
The leaked report says helicopters dropped barrel bombs holding chlorine gas, a prohibited weapon, on the north-west province of Idlib in March 2015.
An earlier report by the Organization for the Prohibition of Chemical Weapons (OPCW) blamed the Syrian government for two other gas attacks in 2015.
The government has not yet commented.
Syria agreed to destroy its chemical weapons in 2013 under the terms of an agreement negotiated between Moscow and Washington.
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The Security Council backed up the agreement with a resolution that if Syria did not comply, it could face sanctions or military action under Chapter 7 of the UN Charter.
The latest findings - the fourth report from the 13-month-long inquiry by the UN and the OPCW - accused government forces of carrying out the toxic gas attack in Qmenas, Idlib governorate, on 16 March 2015.
The conclusions could herald a disagreement between the five veto-wielding powers on the Security Council, correspondents say - with Russia and China against the US, the UK and France.
The OPCW's third report, submitted in August, also accused the so-called Islamic State (IS) group of using sulphur-mustard gas.
At that time, Russia argued that the conclusions against it and the government should not be used a pretext to impose UN sanctions.
The use of chlorine as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013.
The latest report, which was supposed to be confidential but was widely leaked, accused Syrian government forces of using helicopters to drop barrel bombs, which in turn released chlorine gas.
The report said that the helicopter flights could have originated from two bases where the 253rd and 255th squadrons, part of the 63rd helicopter brigade, were based.
Navy helicopters from the 618th squadron were also located at one of the bases.
But the inquiry said it "could not confirm the names of the individuals who had command and control of the helicopter squadrons at the time".
It recommended that those "with effective control in the military units... must be held accountable".
The US ambassador to the UN, Samantha Power, has described the use of chemical weapons as "barbaric" and called for "all states to support strong and swift action".