UN investigators say Syrian forces were behind a chemical weapons attack on civilians in Idlib province in March 2015.

An international inquiry has blamed Syrian government forces for a third chemical weapons attack, according to a confidential report to the United Nations Security Council.

The report, prepared by a joint committee set up by the UN and the Organisation for the Prohibition of Chemical Weapons and seen by Reuters news agency, was presented to the security council on Friday.

The UN experts behind the report said Syrian forces were responsible for a toxic gas attack in the village of Qmenas in Idlib province on March 16, 2015. The committee was unable to determine who was behind two other gas attacks – against Binnish in Idlib province in March 2015 and Kafr Zita in Hama province in April 2014.

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“A joint investigative mechanism was set up by the international chemical weapons watchdog and the UN to investigate reports of chemical attacks in Syria,” Al Jazeera’s Mike Hanna, reporting from the UN headquarters in New York, said.

“Now in its fourth and final report, it says it found a third chemical attack carried out by the Syrian army.”

The UN-led joint investigative mechanism (JIM) reported in late August that Syrian government forces had carried out at least two chemical attacks in 2014 and 2015 and that Islamic State of Iraq and the Levant (ISIL, also known as ISIS) fighters had used mustard gas on the battlefield.

Of the nine total alleged chemical attacks it is considering in its ongoing probe, the JIM has now attributed three to the Syrian government and one to ISIL.

In its fourth report, investigators concluded that there is now “sufficient information” that attack on Qmenas, near Idlib city, “was caused by a Syrian Arab Armed Forces helicopter dropping a device from a high altitude, which hit the ground and released the toxic substance that affected the population”.

Investigators say the substance may have been chlorine gas, based on the symptoms the victims displayed.

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In Kafr Zita, however, the JIM could not confirm that the Syrian army had used barrel bombs to dump toxic substances because “the remnants of the device allegedly used had been removed”, the report said.

Investigators also said that a “canister with traces of chlorine” was found in Binnish, though the container could not be “linked to any of several incident locations identified”.

Hamish de Bretton-Gordon, a chemical weapons adviser to NGOs working in Syria and Iraq, welcomed the findings but said the report should had been made released earlier.

“The fact that it took 18 months for these results to be published is the real issue, and I think the UN need to look at that because having that amount of time before taking action is just not realistic in the current day,” he told Al Jazeera.

Calls for sanctions

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 inquiry’s mandate was extended until October 31 to finish the probe.

“Now this report will go to the security council which will discuss it in a closed session in the coming week, and certainly there is going to be a very heated discussion,” Hanna said.

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“After JIM’s previous report the US and Russia agreed that they would agree between themselves what action to take next.

“But other members of the council, in particular Britain and France, are likely now to push for far more drastic measures to be taken by the security council and certainly there will be intense debate about what the security council is going to do next”, our correspondent said.

Governments in Paris, London and Washington have already called for sanctions against perpetrators of chemical attacks in Syria, including against the government of President Bashar al-Assad.

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.

“To drop these weapons on civilians is absolutely deplorable and reprehensible,” de Bretton-Gordon, the chemical weapons adviser, told Al Jazeera.

“I think the UN should seriously consider a no-fly zone for Syrian helicopters to prevent this from happening in the future.”

Syria agreed to get rid of its chemical stockpile and refrain from making any use of toxic substances in warfare when it joined the Chemical Weapons Convention in 2013, under pressure from Russia.

The Syrian government has also been accused of using chemical weapons in rebel-held areas in Syria in 2016 and investigations into one these occasions are still ongoing.

READ MORE: UN rights chief denounces Aleppo raids as ‘war crimes’

Meanwhile, security concerns forced the UN to delay planned evacuations from Aleppo, the world body said, as Russia extended a truce that was largely holding into a third day on Saturday.

Moscow said it was extending the unilateral “humanitarian pause” in the Syrian government’s Russian-backed assault on opposition-held east Aleppo until 16:00 GMT.

But there was no sign that civilians or rebels were heeding calls to leave, with Damascus and Moscow accusing opposition fighters of preventing evacuations.

The German Chancellor Angela Merkel has demanded an end to air strikes on Aleppo’s residential areas.

Speaking at a meeting of EU leaders in Brussels on Thursday, she described the situation in Aleppo as “barbaric”.

On the same day in Geneva, the UN rights council called for a special investigation into the violence in Aleppo in a resolution fiercely critical of Syria’s government.

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East Aleppo, which the rebels captured in 2012, has been under siege by the army since mid-July and has faced devastating bombardment by the government and Russia since the September 22 launch of an offensive to retake the whole city.

Nearly 500 people have been killed, more than a quarter of them children, since the assault began. More than 2,000 civilians have been wounded.

The scale of the casualties has prompted international outrage, with Washington saying the bombardment amounted to a possible war crime.

Russia announced a halt to its air strikes from Tuesday and the unilateral ceasefire from Thursday.

The Syrian army says it has opened eight corridors across the frontline for the more than 250,000 civilians in rebel-held areas to leave, but so far almost none have taken up the offer.

“There has been no movement in the corridors in the eastern district. For the moment, we haven’t seen any movement of residents or fighters,” said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitoring group.