But Dan Kaszeta, a chemical weapons specialist and managing director of Strongpoint Security, told CNN that the Russian version of events was "highly implausible."
All the nerve agents used in the Syrian conflict so far have been binary nerve agents, he said, which are mixed from different components within a few days of use. This is done because of the difficulties of handling agents such as Sarin, which has a very short shelf life, he said.
"Nerve agents are the result of a very expensive, exotic, industrial chemical process -- these are not something you just whip up," he said.
The idea that the Syrian opposition would be able to build the covert supply chain to make a nerve agent and then would move it around and store it in a warehouse, rather than a bunker, makes no sense, Kaszeta said.
"It's much more plausible that Assad, who's used nerve agents in the past, is using them again," he said.
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Charles Lister, a senior fellow at the US-based Middle East Institute, also rejected the Russian account of what happened as "laughable."
"Are we seriously meant to believe that the opposition has a latent chemical weapons capability, and yet somehow, it has only ever suffered the effects of its own weapons and failed to use them itself?" he said.
What happened in Khan Sheikhoun "is an almost exact replication of what we saw in the summer of 2013," Lister told CNN, adding that it had targeted a key staging area for an opposition push into northern Hama late last month.
Lister also highlighted the issue that all nerve agents known to be in Syria are binary weapons.
"First of all, nobody in their right mind would ever store both both components of a binary nerve agent in the same building. And secondly, even if they were stored together and then targeted, blowing them up would not result in any active nerve agent -- it's chemically impossible," he said.
