Syrian President Bashar al-Assad has said that his country is capable of confronting any external attack, a day after his US counterpart Barack Obama called for military action against him.

The statement came a day after Obama stepped back from his threat to launch an attack unilaterally, instead saying he would consult the US Congress before any such action.

"Syria ... is capable of confronting any external aggression," Syrian state television quoted Assad as saying on Sunday 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." 

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Syria generally refers to rebels fighting to topple Assad as "terrorists".

Faisal Mekdad, Syrian deputy foreign minister, accused Obama of indecision.

"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," he said in Damascus.

The comments came as John Kerry, US secretary of state, said his government had obtained evidence that sarin gas was used in an attack in Syria last month that the US claims killed 1,400 people.

Hair and blood samples provided to the US from first responders on the scene of last month's attack in Damascus "have tested positive for signatures of sarin," Kerry told NBC and CNN television on Sunday.

Kerry said the "case is building'' for a military attack against the regime of Assad. However, the evidence has yet to be presented to the public.

The Assad regime is known to have stockpiles of sarin, a deadly nerve agent, although the regime has claimed that rebels have used the substance in attacks. 

Obama said on Saturday that the US had presented a "powerful case" linking the Assad regime to the attack in Ghouta on August 21, and that the US military was prepared to launch a "limited" strike.

Congress is due back from its summer recess on September 9.

A French government source meanwhile told the AFP news agency that officials would soon declassify secret defence documents detailing Syria's chemical arsenal.

The comment came after the French Journal du Dimanche newspaper said Syria had 1,000 tonnes of chemicals including sarin and mustard gas, and was developing a powerful agent that was far more toxic than sarin.

"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."

Source: Al Jazeera and agencies