WASHINGTON, Sept 1: US Secretary of State John Kerry said on Sunday that samples collected from the sites of the Aug 21 chemical weapons attack in Syria have tested positive for Sarin gas.

In a series of interviews to various US television channels, Secretary Kerry claimed that medical volunteers, who were among the first to arrive at an attack site in East Damascus, had collected the samples.

“Blood and hair samples that have come to us through an appropriate chain of custody from east Damascus from first responders — it has tested positive for signatures of Sarin,” he told CNN. “Each day that goes by, this case is even stronger.”

In another interview to NBC, Secretary Kerry said the use of chemical weapons puts Syrian President Bashar el-Assad in the same category as the world’s most bloody dictators.

“Bashar Assad now joins the list of Adolph Hitler and Saddam Hussein [who] have used these weapons in time of war,” he said.

Sarin is a colourless, odourless liquid which quickly turns into a gas. It attacks the nervous system, choking the victim to a painful death in one minute after the exposure.

A victim’s clothing can release Sarin for about 30 minutes after contact, which can lead to exposure of other people. UN resolution 687 classified Sarin as a weapon of mass destruction and the Chemical Weapons Convention of 1993 outlawed its production and stockpiling.

More than 1,400 people, including 400 children, were killed in the Aug 21 chemical weapons attack in East Damascus.

The United States claims that the Syrian government used the weapons to subdue rebels in an opposition-controlled neighbourhood. The Syrian government denies the charge and has blamed the rebels for the attack.

In interview to ABC News, Mr Kerry said President Barack Obama could act even if Congress did not back him, but “we are not going to lose this vote”, he added.

In a statement he read at the White House Rose Garden on Saturday, President Obama declared that he would take a military action against the Syrian government but with congressional approval. He sent the matter to Congress “for a debate and a vote”.

Congress resumes on Sept 9 after a summer break and congressional leaders have promised to start the debate early next week.

While both Republicans and Democrats have welcomed Mr Obama’s decision to seek congressional assent, some also warned there’s no assurance that Congress would approve his plan.

Senator James Inhofe, the top Republican on the Senate Armed Services Committee, told Fox News he didn’t think the use of force would be approved. “Our military is so degraded right now,” he said.

Congressman Peter King, another Republican who chairs a House subcommittee on counter-terrorism, also warned that it would “be difficult to get the vote through” a Republican-dominated Congress. “If the vote was today it would probably be a ‘No’ vote. The president has not made the case.” he said.

The congressman said he would vote for the action but he was among a few who would do so as most lawmakers were reluctant to back Mr Obama.

Secretary Kerry, who spent more than 30 years in Congress before joining the administration, disagreed.

“I don’t believe that my former colleagues in the 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,” he told NBC.

Another Democrat, Senate Majority Leader Harry Reid, voiced his support for military action in Syria and said the Senate would vote in favour of the president’s decision.

Republican Senators John McCain and Lindsey Graham said they would support the president if he agrees to do more than launching “just surgical strikes”. They want the US administration to help rebels topple the Assad government.

Some Republican lawmakers suspected that the White House might have decided not to use force against Syria and was now seeking a congressional stamp on its decision.

“I hope this is not a case of them having second thoughts and using Congress as a foil,” said Senator Bob Corker, the senior Republican on the Senate Foreign Relations Committee. Mr Corker said he sought and received assurances from the White House that the president was fully committed to pursuing the military options.

Mr Obama’s decision to go to Congress followed a vote in the British parliament last week which rejected Prime Minister David Cameron’s request for authorising the use of force against Syria.