He suggested President Bashir al Assad had joined the ranks of Adolf Hitler and Saddam Hussein in using the banned nerve agent against his own people.
In response, the Syrian President goaded the US administration. Reacting to the surprise decision by Barack Obama to seek Congressional support for any military strike, Mr Assad hailed a "historic American retreat" and pledged his government would repel "any external aggression".
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In Washington, Mr Kerry explained how in the past 24 hours, tests on emergency workers in Damascus had shown hair and blood samples had tested positive for signatures of Sarin, building the case against Syria. He said he was confident Congress, meeting next Monday, would back military action, saying his colleagues would do what was right.
In Paris, responding to Mr Obama's decision to consult Congress, France's Interior Minister Manuel Valls made clear the French government would wait, noting: "France cannot go it alone; we need a coalition."
The developments came as UK Government ministers made clear they had no intention of reconsidering any renewed attempt to persuade MPs towards Britain's involvement in military action.
George Osborne declared: "Parliament has spoken." He pointed out that the Tory and Liberal Democrat rebels had a deep scepticism about military involvement.
The Chancellor added: "I don't think another UN report, or whatever, would make the difference. Of course, I wanted us to be part of a potential military response. Now that is just not going to be open to us now."
His remarks were echoed by William Hague, who insisted Labour would have to "play a less partisan and less opportunistic role" but he could see no "immediate possibility" of circumstances changing enough to secure support from Westminster.
Among those who suggested MPs might have to reconsider their opinion on military involvement in light of the US intelligence was Sir Malcolm Rifkind, the former Foreign Secretary.
He said because of the new intelligence Ed Miliband and David Cameron "ought now really to get together and say - look, if we can now agree the evidence is compelling, then Parliament ought to have the opportunity to debate the matter again".
Meantime, the SNP condemned the UK Government after reports that it had allowed the sale of chemicals to Syria, which can be used to make Sarin.
The licences, it noted, were only revoked when the EU imposed sanctions on the Assad regime last summer.
Angus Robertson, the Nationalist leader at Westminster, said: "This is utter hypocrisy from the UK Government; deploring chemical weapons in public whilst approving the sale of items needed to make them."
He added: "I will be raising this at Westminster as soon as possible to find out what examination the UK Government made of where these chemicals were going, and what they were to be used for."