The United Nations has accused the Syrian government of deliberately attacking an aid convoy near Aleppo last September, and labeled a forced evacuation of opposition-held parts of the east of the city as a “war crime”.

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In a report covering the capture of Aleppo by forces supporting the Syrian regime, the UN also accused Damascus of repeatedly using chemical weapons and cluster munitions, and systematically destroying hospitals.

The UN commission of inquiry for Syria said war crimes were committed by both sides from July until the city fell on 22 December.

However, it levelled extensive criticism at the government of Bashar al-Assad, which was strongly supported by Russian jets and Iranian-backed militias throughout the campaign.

A centrepiece of the investigation was the accusation that Syrian jets had conducted a series of airstrikes on 19 September that targeted an aid convoy, which had been permitted by regime officials to deliver food and medicines to the Urum al-Kubra area west of Aleppo. The evidence strongly suggested that the attack in which 20 civilians and aid workers were killed was “meticulously planned and ruthlessly carried out”, it said.

The destruction of most of the convoy and the subsequent suspension of further aid deliveries served as a “starve or surrender” policy pursued by the regime in east Aleppo and other opposition-held areas of Syria.

Facebook Twitter Pinterest Wreckage of Syrian Red Crescent aid vehicles seen in Aleppo in September 2016. Photograph: Anadolu Agency/Getty Images

A previous UN inquiry found that it could not determine who had carried out the airstrike, which came amid widespread international pressure for aid to be allowed into rebel-held areas. However, the new report laid the blame squarely at the feet of the Syrian regime, detailing three phases of attacks, the first of which involved barrel bombs – a weapon not deployed by the Russian air force.

“First helicopters dropped barrel bombs, which struck the warehouse and a family home nearby,” the report said, citing testimonies from some of the 291 people interviewed by the UN inquiry team.

Scientific evidence and satellite imagery supported the findings. “Subsequently, planes, described by several witnesses as Sukhoi jets, carried out attacks, killing several aid workers. Lastly the aircraft fired machine guns at survivors.”

The report also said there was conclusive evidence that Syrian forces had dropped toxic industrial chemicals, including chlorine on opposition communities throughout the last year, causing hundreds of casualties. It did not attribute blame on Russian forces, which – along with Iran – helped turn the tide for Assad soon after Vladimir Putin ordered Russia’s intervention in September 2015.

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On Tuesday, Russia used its veto for a seventh time to protect Syria from UN security council sanctions over allegations of chemical weapons use throughout the six year war. Syrian forces have repeatedly been accused of using chemical weapons – including sarin, the nerve agent that killed an estimated 1,300 people in the opposition-held area of Ghouta, near Damascus, in August 2013.

Western intelligence officials believe that sarin was again used last December in the eastern Hama region – an area held by Isis, which days earlier had seized the historic city of Palmyra for a second time.

At least 92 people were killed and about 300 left incapacitated by airstrikes in five small villages, medical organisations confirmed. However, unlike the attacks near Damascus, intelligence agencies were unable to retrieve biological samples, from either the blood of victims or contaminated soil.

Facebook Twitter Pinterest A damaged truck in Aleppo in September 2016, the morning after the aid convoy it was part of was hit by a airstrike. Photograph: Omar Haj Kadour/AFP/Getty Images

Vladimir Safronov, the deputy Russian envoy to the UN, said Moscow was sceptical of the results of a joint investigation – in which it had participated – which found that Syrian government forces had carried out at least three chlorine gas attacks. It also found that Isis had been responsible for at least one attack involving mustard gas.

The UN report also detailed the systematic destruction of healthcare facilities in east Aleppo. It found that no hospitals were functioning there at the time the last residents fled. During the recapture of rebel-held parts of the city, pro-regime forces had arrested doctors and aid workers and committed reprisal executions, the report claimed.

The report claimed the regime and the opposition had each committed war crimes by forcing the evacuation of the city for “strategic reasons” and “not for the security of civilians, or imperative military necessity”. The report concluded that the Aleppo evacuation agreement (which was overseen by the International Committee of the Red Cross) amounted to the “war crime of forced displacement”.

Opposition forces in east Aleppo, made up from both moderate and jihadi groups, were condemned for shelling government-held areas of west Aleppo in attacks that killed and wounded dozens of people. Rebel groups had also prevented some civilians from fleeing east Aleppo, used some communities as “human shields” and attacked a Kurdish neighbourhood.