WASHINGTON - US Secretary of State John Kerry claimed Sunday that United States has new evidence showing that Damascus used the nerve gas sarin against its own people.

Kerry said on Sunday talk shows that the US has received hair and blood samples that have "tested positive for signatures of sarin," a deadly nerve agent first developed in Germany in the 1930s.

The secretary of state spoke a day after President Barack Obama, wilting under domestic and international pressures, backed away from what looked like an imminent military action against Syria, saying his administration would seek Congressional endorsement before hitting the Middle East country. He spoke with great deal of passion in an attempt to convince American people of the Obama administration's case on punishing Syria for the alleged use of chemical weapons near Damascus on August 21.

With many lawmakers openly skeptical, Kerry predicted that Congress would not “turn its back” on its obligation to uphold international norms against the use of chemical weapons.

Congress is scheduled to meet for nine legislative days in September. In addition to debate over military intervention in Syria, Congress will also have to extend government spending authority, which expires at the end of the month.

"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, and hair samples and blood samples have tested positive for signatures of sarin," Kerry said on NBC's Meet the Press.

On CNN's State of the Union, Kerry laid out the same case, and noted that the new evidence came not from United Nations inspectors but from independent channels. Kerry said the new evidence boosts President Obama's call for Congress to approve military action against Syria. "I believe the case for action will get stronger by the day," he said in an apparent attempt to shore waning support for military strikes on Syria.

The administration continues to avoid saying that there is 100 per cent proof of its case against Syrian President Bashar Assad. "The word 'slam-dunk' should be retired from American national security issues," Kerry told NBC host David Gregory.

The US Centers for Disease Control and Prevention describes sarin as "a human-made chemical warfare agent classified as a nerve agent. Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents." Sarin was developed as a pesticide, and it works by blocking an enzyme that serves as the "off-switch" for the body's glands and muscles, sending normal bodily systems into constant activity. Eventually victims can no longer breathe.

On Saturday, President Obama said he had decided the United States should launch a military attack against Syria in retaliation for an Aug 21 chemical weapons attack in the Damascus suburbs. But he said he would first ask Congress to approve the use of military force. The House and Senate will both vote on the resolution soon after their return from recess Sept 9. Kerry said in several Sunday appearances that Obama believes congressional approval will make the nation stronger and give the Pentagon more flexibility in pursuing military options against Syria.

"He believes we need to move, he's made his decision. Now it's up to the Congress of the United States to join him in affirming the international norm with respective enforcement against the use of campaign weapons," Kerry said.

He said the White House does not expect Congress to vote against the use of force, but he also indicated that the president is not waiving his right to act if Congress does not."He has the right to do that no matter what the Congress does," Kerry said on CNN.

The UN team of chemical weapons experts spent four days near the site of the alleged attack and brought an extensive collection of forensic samples and other evidence to The Hague for testing in European laboratories.

Agencies add: Syrian President Assad however remained defiant on Sunday saying Syria "is capable of facing up to any external aggression."

Obama will be relatively confident of winning a vote in the Democrat-controlled Senate. Influential Republican Senator John McCain has also argued for military action, but wants it to go further and topple Assad to end a brutal war which has claimed more than 100,000 lives since erupting in March 2010.

Speaking in Cairo on Sunday, Saudi Foreign Minister Saud al-Faisal urged Arab countries to back calls by the Syrian opposition for strikes on the Damascus regime. Arab states must echo demands by the "legitimate" representatives of the Syrian people for "help from the international community to put an end to the bloodbath" in Syria, he said, referring to the opposition.

The Saudi foreign minister did not explicitly mention a call by US President Barack Obama to launch punitive strikes on Syria for allegedly unleashing chemical weapons on its citizens last month, which according to Washington includes Sarin gas and killed hundreds.

But he told a news conference in Cairo that the international community must stop "the aggression against the Syrian people before these people perish".

The Saudi foreign minister urged his counterparts in Arab League to back the Syrian opposition beyond condemnations of regime atrocities.

"Condemnations are not enough," he said.