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They included the April 2018 attack on Douma, in the Eastern Ghouta, when a Syrian air force helicopter is believed to have dropped two cylinders filled with highly concentrated chlorine gas on two apartment buildings, killing at least 43 people.

Russia also negotiated ceasefires and deals for the surrender of rebel-held towns and cities, such as Douma in 2018.

In Aleppo, Douma and elsewhere, the regime forces besieged rebel-held areas, cutting off food and medicine supplies, and proceeded to bomb them until armed opposition groups surrendered.

"Russia's crimes here were indescribable," says Ahmed Taha, a rebel commander in Douma, six miles north-east of the capital Damascus.

Entire residential districts in Douma now lie in ruins after some of the fiercest fighting in Syria's almost 14-year civil war.