Kerry also voiced confidence on TV interviews that the US Congress "will do the right thing" after President Barack Obama's request to take military action against the Syrian government.
Hair and blood samples provided to the US from first responders on the scene of last month's attack in the Syrian capital "have tested positive for signatures of sarin," Kerry told NBC and CNN television.
President Bashar al-Assad said on Sunday Syria was capable of confronting any external attack after Obama said there should be a military strike on Syria.
"Syria … is capable of confronting any external aggression," state television quoted Assad as saying at a meeting with Iranian officials. "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 generally refers to rebels fighting to topple Assad as "terrorists".
However, Syria's opposition coalition called on on the US Congress to grant approval for military action against President Bashar al-Assad and said any intervention should be accompanied with more arms for the rebels.
"The Syrian Coalition believes any possible military action should be carried out in conjunction with an effort to arm the Free Syrian Army. This will be vital in restraining Assad and ending the killing," the coalition said in a statement.
France ready for war
France will soon declassify secret defence documents detailing Syria's arsenal of chemical weapons in defiance of international conventions, a government source said Sunday.
The comment came after the Journal du Dimanche weekly said French intelligence agents had compiled information showing that some of the weapons had been stockpiled for nearly 30 years.
The arsenal included over 1 000 tonnes of chemical agents, the paper said. "The citations from the notes are correct," the source said.
"The government plans to make public the declassified documents on the Syrian chemical arms programme." According to the Sunday newspaper, the arsenal included Sarin and mustard gas.
The secret documents showed that Syrian scientists had also worked to develop a powerful agent that was far more toxic that Sarin.
French President Francois Hollande, who appears determined to launch imminent strikes on Syria, is hamstrung by the US, which has delayed taking action, and growing domestic opposition to military intervention.
Hollande, who unilaterally intervened in Mali to prevent Islamists from proceeding south towards the country's capital, has repeatedly voiced his determination to "punish" President Bashar al-Assad's regime for alleged chemical weapons attacks on August 21.
France has been catapulted to the status of Washington's main ally in the Syria crisis after the British parliament in a shock move rejected plans for military action mooted by Washington. But Paris risks being seen as the "trailer" to the American vehicle in case of an intervention, former French prime minister Francois Fillon warned over the weekend.
He said France should act "responsibly" and not follow anyone into an attack "even if they are our friends and allies, the Americans."
"The region is a powder-keg," Fillon warned.
Syrian Deputy Foreign Minister Faisal Muqdad on Sunday branded the French government as "irresponsible" and accused Hollande and foreign minister Laurent Fabius of duping "the French people to justify the failed policies against Syria." –Reuters –AFP