An international inquiry conducted by United Nations and the global chemical weapons watchdog Organization for the Prohibition of Chemical Weapons, has declared that the Syrian government carried out a third chemical attack in the conflict-wracked nation last year.

The group's fourth confidential report during a 13-month investigation sent to the UN Security Council late Friday shows there is “sufficient evidence” to confirm the government’s role in the March 16, 2015 strike on Qmenas in Idlib governorate.

The leaked report also says that helicopters dropped barrel bombs holding chlorine gas, a prohibited weapon, on the north-west province of Idlib in March 2015, reported BBC.

Syrian government has not yet commented on the report.

In August, the UN team blamed President Bashar Assad's government for using chlorine gas in two attacks and Islamic State fighters for using mustard gas in one attack.

The team said at that time that three other attacks indicated possible government involvement.

It said a device, dropped from a high altitude, "hit the ground and released the toxic substance that affected the population." Witnesses and hospital staff identified the smell and symptoms of chlorine gas.

The panel said "it is crucial to hold those who use or intend to use chemicals as weapons accountable for their acts, as it is fundamental to deter all those who continue to believe that there is something to be gained in the use of toxic chemicals as weapons."

The US, Britain and France have already called for sanctions against perpetrators of chemical attacks in Syria, including against the regime in Damascus.

But the Syrian government has been shielded by its ally Russia, which has questioned the JIM findings and said the evidence is not conclusive enough to warrant sanctions.

Russia's UN Ambassador Vitaly Churkin indicated that Moscow will oppose any sanctions.

The Security Council, which is expected to discuss the report on Thursday, 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.

Syria had in 2013 agreed to destroy its chemical weapons under the terms of an agreement negotiated between Moscow and Washington.

Also the use of chlorine as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013.

On the barrel bombs, 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".