The US secretary of state says hair and blood samples prove that the Assad regime used deadly nerve gas in an attack on rebel positions.
Mr Kerry spoke after President Barack Obama put the brakes on the threat of an immediate military strike against Syrian government forces, saying there would be a debate and a vote in Congress first.
In a series of interviews on Sunday, Mr Kerry stopped short of saying that the president would authorise the use of force without the approval of Congress, but he told ABC: “We are not going to lose this vote.”
Mr Kerry said: “The case hasn’t changed and the case doesn’t change at all. The rationale for a military response is as powerful today.
“This case is going to build stronger and stronger,” he told NBC, adding that “the people of America should be celebrating that the president is not acting unilaterally”.
The secretary of state said that Assad “has now joined the list of Adolf Hitler and Saddam Hussein” in deploying chemical weapons against his own people.
He went on: “In the last 24 hours we have learned through samples that were provided to the United States, that have now been tested, from first responders in east Damascus, and hair samples and blood samples have tested positive for signatures of sarin.”
Asked if the US obtained its information from UN weapons inspectors who visited Syria, Kerry said: “No, it is independent. … But it is confirmation of the signatures of sarin.”
Asked repeatedly what Obama would do in the event that Congress refuses to give its consent, Kerry said, “The president has taken his decision.
“I think this is a smart decision by the president. … He is not trying to create an imperial presidency. I believe that in the end, Congress will do what is right.”
Administration officials have said that Obama appeared set on ordering a strike until Friday evening. After a long walk around the White House grounds with Chief of Staff Denis McDonough, the president told his aide he had changed his mind.
Also speaking on Sunday, President Bashar al-Assad said: “Syria … is capable of confronting any external aggression.
“The American threats of launching an attack against Syria will not discourage Syria away from its principles … or its fight against terrorism supported by some regional and Western countries, first and foremost the United States of America.”
Syria’s deputy foreign minister Faisal Mekdad accused Obama of indecision, saying: “It is clear there was a sense of hesitation and disappointment in what was said by President Barack Obama yesterday. And it is also clear there was a sense of confusion as well.”