Of course he wishes not to confirm the event, if he did, then the vow of retaliation would be de-legitimized on every front. Not only that but the interests of Imperialist and Neo-Liberal capitalists would be squandered at not being able to abuse a contingent event like this.
But the funny part is that instead of finding evidence of the Syrian Government doing it, they would find the "Opposition" rebels and imperialist forces committing the action. Thus they would find the event to be a false flag meant to provoke military action against Syria.
Truly the Imperialists and Neo-Liberals are retreading on old ground and reusing every trick in the book to weaken Assad as much as possible and sap every little penny they can before they leave. What a world we live in.