Here is the quote
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"The Russian Defence Ministry spokesman, Maj. Gen. Igor Konashenkov, said in a statement that Russian military assets registered the strike on a weapons depot and ammunition factory on the town’s eastern outskirts. Konashenkov said the factory produced chemical weapons that were used in Iraq.
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The case stands against the rebels themselves or whoever owned and ran this ammunition factory. 
So, it is not the Assad regime (which is, no doubt, criminal not lesser than its fighting foes) who committed the crime.
And staying on the subject of crimes in Syria and Iraq. How is the investigation of the recent US airstrike that killed more civilians than in this sad episode going? The last time I heard about it was a few days ago - and then complete silence. If that was the Russian or Syrian air force, I am sure we would have continued to hear about the happening.