United Nations war crimes investigators have formally blamed the Syrian government for a sarin attack on the town of Khan Sheikhun in April that killed at least 83 people and wounded almost 300 more.

The international commission of inquiry on Syria, which is run by the UN, said it had compiled extensive evidence that the Syrian air force had carried out an airstrike that released the nerve agent. It dismissed claims by Damascus and Moscow that the carnage was the result of a bomb striking an opposition chemical weapons depot on the outskirts of the northern town as “fabricated”.



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In the most conclusive findings to date from investigations into chemical weapons attacks during the the country’s six-year civil war, it found that Syrian forces had used chemical weapons 20 times, including in the deadly attack on Khan Sheikhun that led to US airstrikes.

In the report, produced for the UN human rights council, the commission said it had conducted 43 eyewitness interviews and analysed satellite imagery, key timings on the day of the attack and bomb remnants that it had partially matched to the type of munitions that the former Soviet Union is known to have supplied to the Syrian military.



The report found that a Sukhoi 22 jet, which is only flown by the Syrian airforce, had carried out four airstrikes at 6.45am on 4 April. Three bombs carried conventional explosives, but one, which struck a road, carried the deadly nerve agent, which was carried as far as 600 metres away on a gentle wind.



“Weather conditions at 6.45am on 4 April were ideal for delivering a chemical weapon,” the report said. “The wind speed was just over 3km/h, with no rain and practically no cloud cover. Under such conditions, the agent cloud would have drifted slowly downhill following the terrain features at the location.”



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The Assad government has repeatedly denied using chemical weapons, but the report said the Syrian regime’s version of events, that an unknown weapons depot had been hit, was “extremely unlikely”. It said sarin stored in such circumstances would have mostly burned off or been absorbed by rubble, and that there was no evidence of the building being contaminated.



A visit by the Guardian to Khan Sheikhun two days after the attack revealed that the site officials claimed had been hit had been empty for many months, and contained only animal feed and a volleyball net. Witnesses described the frantic aftermath of a series of airstrikes, which overwhelmed the limited capacity of local medics and rescue workers.



Claims made by locals on the ground that the airstrikes had taken place at around 6.45am were endorsed by the commission, which said the Russian and Syrian version of events – that the attack had taken place at 11.30 that morning – was not supported by facts.

“The commission has not, however, found any evidence to support the claim that … armed groups had a weapons depot in the area where the chemical bomb impacted. The evidence laid out above overwhelmingly indicates that the sarin gas was released at around 6.45am. All evidence available leads the commission to conclude that there are reasonable grounds to believe Syrian forces dropped an aerial bomb dispersing sarin in Khan Sheikhun.



“The use of sarin in Khan Sheikhun on 4 April by Syrian forces constitutes the war crimes of using chemical weapons and indiscriminate attacks, and violation of the prohibition on the use of weapons designed to cause superfluous injury and unnecessary suffering.”



The commission said it had documented 25 instances of chemical attacks in Syria between March 2013 and March 2017, 20 of which had been carried out by government forces and had mainly targeted civilians. In the other cases, the perpetrators had not been identified. It said an attack in the nearby town of Latamneh a week earlier had left dozens of victims with similar symptoms to those seen in Khan Sheikhun, but none had died.

The attack was previously identified as having used sarin, an odourless nerve agent, but the fact-finding mission of the Organisation for the Prohibition of Chemical Weapons did not say who carried it out.