Secretary of state says the US has evidence that sarin gas was used in chemical attacks, as the Obama administration seeks to persuade congressional sceptics of military action

The US has evidence that sarin nerve gas was used in chemical attacks outside Damascus last month and could go ahead with military strikes against Bashar al-Assad's regime even without the backing of Congress, the US secretary of state, John Kerry, has said.

A day after Barack Obama vowed to put any intervention in Syria to a vote of both the Senate and House of Representatives, Kerry said the administration was confident of winning a motion of the kind that David Cameron unexpectedly lost last week. "We don't contemplate that the Congress is going to vote no," Kerry said, but he stressed the president had the right to take action "no matter what Congress does".

In a round of appearances on the Sunday political shows in the US, Kerry said the evidence of sarin came from samples from first responders who had helped victims of the attacks. "[We have] blood and hair samples that have come to us through a secure chain of custody from east Damascus – it has tested positive for signatures of sarin. So each day that goes by this case is even stronger," he said.

Kerry said America's evidence for the use of sarin nerve gas had not come from the UN. He gave no further details of the source of the samples, or where or when they had been tested.

He said the Obama administration's clear preference was to win a vote in Congress, which could come as early as next week, after politicians return from their summer recess on 9 September. He could "hear the complaints" about presidential abuse had Obama not gone to Congress, and its backing would give any military action greater credibility: "We are stronger as a nation when we act together." But he added: "America intends to act."

On Sunday, Britain definitively ruled out any involvement in military strikes against Syria even if further chemical attacks take place.

William Hague, the foreign secretary, said Britain would offer only diplomatic support to its allies. "Parliament has spoken. I don't think it is realistic to think that we can go back to parliament every week with the same question having received no for an answer."

His remarks were echoed by the chancellor, George Osborne, who said he did not think more evidence or more UN reports would have convinced the MPs who voted against intervention. He also ruled out a rerun of the vote.

Syrian opposition figures have reacted angrily to what they perceive as America's delay in striking against Assad. While the Obama administration insists that military intervention would be a punishment for the chemical weapons attack and a deterrent against future incidents rather than an attempt at regime change, many in the fractured opposition hope it will tip the military balance in their favour after a two and a half year civil war that has killed about 100,000 people.

Samir Nishar, of the opposition Syrian National Coalition, called Obama a "weak president", according to CNN.

The mood among government forces in Syria was triumphant after Obama's speech, with claims in the state-run media that the west had backed down because it was afraid of a confrontation.

Syria's permanent representative to the UN, Bashar al-Jaafari, said Obama and Cameron had decided to seek approval before military action because they were looking for a way out after banging the drums of war, the official news agency Sana reported. Jaafari said the two leaders had "climbed to the top of the tree and don't know how to get down".

The UN said it had asked the chemical weapons team to expedite its report into the use of the weapons. "The secretary general took note of the announcement by President Obama yesterday on the referral to Congress. He regards it as one aspect of an effort to achieve a broad-based international consensus on measures in response to any use of chemical weapons," UN spokesman Martin Nesirky said.

"Use of chemical weapons will not be accepted under any circumstances," he added, asking that the investigation mission "should be given an opportunity to succeed".

Kerry also suggested that Obama will not limit US involvement in Syria's civil war to cruise missile strikes provoked by the use of chemical weapons. The administration "may even be able to provide greater support to the opposition", he said. Obama began providing weapons to Syrian rebels after determining earlier this year that Assad had carried out a smaller-scale chemical attack.

But there is a deep reluctance within the US military to bless even a one-off military strike. General Martin Dempsey, the chairman of the joint chiefs of staff and a multi-tour veteran of Iraq, has voiced such fears for more than two years.

Some leading figures on Capitol Hill predicted that Obama would win. "At the end of the day, Congress will rise to the occasion," Representative Mike Rogers, the chairman of the House intelligence committee, told CNN.

But others were less sure. Senator Rand Paul, a libertarian Republican, put the chances of an authorisation vote in the House of Representatives at 50-50. "I think the Senate will rubber stamp what he wants but the House will be a much closer vote," he told NBC.

Former Republican presidential candidate John McCain said the administration needed a more decisive plan to topple the Assad regime. But he warned against the possibility of Congress defying the president. "The consequences of a Congress of the United States overriding a decision of the president of the United States on this magnitude are really very serious," he told Face the Nation on CBS.

Meanwhile Labour MP Thomas Docherty, a member of the House of Commons' committee on arms export controls, criticised the government after it emerged that British firms were granted licenses in January 2012 to sell chemicals to Syria that could be used to make nerve gas including sarin.

The chemicals never left the UK as the European Union imposed tough new sanctions on Assad's regime and the licenses were revoked.

But Docherty has written to the business secretary Vince Cable demanding to know more details about the deals.

In his letter he asked: "How many licenses for the export of chemicals have been issued not only in the period in question but in the wider period since the start of the Syrian civil war?"