WASHINGTON: The United States has proof sarin gas was used in a Damascus attack, Secretary of State John Kerry said Sunday, as he urged Congress to vote for military action against the Syrian regime.

Hair and blood samples given to the United States from emergency workers on the scene of last month's attack in the Syrian capital have showed signs of the powerful sarin nerve gas, Kerry told NBC and CNN television.

In what he called “a very important recent development... in the last 24 hours, we have learned through samples that were provided to the United States and that have now been tested from first responders in East Damascus, (that) hair samples and blood samples have tested positive for signatures of sarin,” Kerry told NBC's Meet the Press.

“Each day that goes by, this case is even stronger. We know that the regime ordered this attack. We know they prepared for it. We know where the rockets came from. We know where they landed,” he added on CNN.

“We know the damage that was done afterwards. We've seen the horrific scene all over the social media, and we have evidence of it in other ways, and we know that the regime tried to cover up afterwards.”

Kerry blitzed the Sunday morning television talk shows to relaunch his bid to build the case for US military strikes in Syria after President Barack Obama called for Congress to vote to authorize action.

He urged his former colleagues in Congress to give Obama a green-light for strikes against the regime of President Bashar al-Assad.

In a huge political gamble, Obama has committed the fate of US action to lawmakers, lifting the threat of immediate strikes.

Obama said he had decided an August 21 chemical weapons attack on a Damascus suburb that Washington says killed more than 1,400 people was so heinous that he would respond with a limited US military strike.

But, in a move which could reshape the balance of power between Capitol Hill and the presidency, he said he believed it was important to secure support from Congress to wage war.

Obama will be relatively confident of winning a vote in the Senate, which is controlled by Democrats and includes a number of Republicans, like Senator John McCain, who have argued for military action against Syria.

But it would be hazardous to predict how the vote will go in the Republican-controlled House of Representatives, which will debate Syria as soon as it comes back into session on September 9.

Kerry told NBC he believed the call for action would be approved by Congress.

“I do not believe the Congress of the United States will turn its back on this moment... I believe Congress will pass it,” he said.

“I don't believe that my former colleagues in the United States Senate and the House will turn their backs on all of our interests, on the credibility of our country, on the norm with respect to the enforcement of the prohibition against the use of chemical weapons, which has been in place since 1925,” Kerry said.

“The Congress adopted the Chemical Weapons Convention. The Congress has passed the Syria Accountability Act. Congress has a responsibility here too.”