John Kerry advanced what he called a "clear and compelling" case that Syria was responsible for a chemical attack that killed nearly 1,500 people, in a statement on Friday that made clear the US was on the verge of military strikes against the Assad regime.

Speaking in a blunt terms, the US secretary of state branded the Syrian president, Bashar al-Assad, a "thug and a murderer", and said the United States could not stand by and let a dictator get away with such serious crimes.

"History will judge us all extraordinarily harshly if we turn a blind eye to a dictator's wanton use of weapons of mass destruction," Kerry said in a briefing to reporters at the state department in Washington.

Signals of an impending attack continued to build on Saturday. UN weapons inspectors left their Damascus hotel and their convoy headed via the Masnaa border crossing to neighbouring Lebanon, where they were seen arriving at Beirut international airport to take flights out of the region, according to Reuters and the Associated Press.

Speaking soon after Kerry, Barack Obama said that he was considering "limited, narrow action" against Syria. "We can not accept a world where women and children and innocent civilians are gassed on a terrible scale," he said.

Pointedly, Kerry made no mention of the decision by the British government to pull out of the coalition, after prime minister David Cameron lost a crucial vote in the House of Commons on Thursday. Instead, Kerry referred to France as the "oldest ally" of the US, after President François Hollande pledged support for military action against Syria.

As Kerry spoke, the White House released an unclassified four-page dossier. The assessment said the US intelligence community had "high confidence" that Assad's forces were behind the attack, which it said killed at least 1,429 Syrians, including at least 426 children.

Obama said the US did not intend to be dragged into Syria's civil war. "We're not considering any open-ended commitment," he said at a photo opportunity with Baltic leaders. "We're not considering any boots-on-the-ground approach."

The president conceded that many people, himself included, were "war-weary" after a decade of US military interventions, but added: "A lot of people think something should be done – but nobody wants to do it."

An attack could happen as soon as Saturday when UN weapons inspectors are due to leave Syria after their mission in the country was apparently cut short amid expectations of an attack. The UN secretary general, Ban Ki-moon, told security council members on Friday evening that it could be two weeks before the final results of their analysis is ready.

Late on Friday a sixth US warship joined five US cruise missile destroyers in the eastern Mediterranean. Officials said the USS San Antonio, an amphibious ship with several hundred US Marines on board, had arrived in the region for a different reason but given the current situation it was being kept near the destroyers "as a precaution", the Reuters news agency reported.

On Thursday the White House said any strikes would be "discrete and limited".

Kerry said there would be no boots on the ground and the attack would not be open-ended, "and it will not assume responsibility for a civil war that is already under way". He described a "limited and tailored response that a despot's flagrant use of chemical weapons will be held responsible".

He insisted the impending military action would not be a similar to conflicts in Libya, Afghanistan or Iraq, saying of the 2003 invasion of the last named: "We will not repeat that moment."

However, in a line reminiscent of George W Bush's "axis of evil", Kerry specifically mentioned a host of US enemies, saying Iran could be "emboldened" if the US did not act.

"It is about Hezbollah and North Korea and every other terrorist group that might ever again contemplate the use of weapons of mass destruction. Will they remember the Assad regime was stopped from those weapons' current or future use? Or will they remember that the world stood aside and created impunity?"

Kerry portrayed taking tough action as a matter of US credibility, saying other countries that might use chemical weapons were watching. "They want to see whether the United States and our friends mean what we say," he said. "It matters deeply to the credibility and the future of the United States of America and our allies."

The secretary of state, who along with Obama has been involved in an intense diplomatic offensive to garner support for the administration's Syria policy, sought to cast the planned action as having broad support. He mentioned an Arab League statement and quoted statements from the leaders of Australia and France.

In his statement Kerry gave the most detailed assessment yet of what happened on 21 August. He said Assad's forces had the largest stockpile of chemical weapons in the Middle East and had used them several times this year. The regime wanted to clear problematic Damascus suburbs of opposition forces and had grown "frustrated", he said.

"We know that three days before the attack the Syrian regime's chemical weapons personnel were on the ground in the area making preparations," Kerry said.

He claimed Syrian forces took precautions such as putting on gas masks before the attacks.

"We know that these were specific instructions. We know where the rockets were launched from and at what time; we know where they landed and when. We know rockets came only from regime-controlled areas and went only to opposition-controlled or contested neighbourhoods." Thirty minutes later "all hell broke loose" on social media, Kerry said.

Kerry chose highly emotive language to describe the aftermath of the attacks, painting a vivid scene of "twitching bodies" and victims "foaming at the mouth", all captured in video posted online. "Instead of being tucked safely in their beds at home we saw rows of children, lying side by side, sprawled on a hospital floor – all of them dead from Assad's gas, and surrounded by parents and grandparents who had suffered the same fate."

The secretary of state sought to reassure the public that the intelligence, which has come under growing scrutiny in recent days, was reliable. "This is common sense," he said. "This is evidence. These are facts."

Kerry added that it was in the interests of the world to punish Assad but repeatedly cast the impending action as a matter of US credibility. "If we choose to live in a world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity, even after the United States and our allies said no, and then the world does nothing about it, there will be no end to the test of our resolve, and the dangers that will flow from those others who believe they can do what they will."

Meanwhile, senior administration officials pressed the case against Syria in a telephone briefing for journalists. "I don't think there's any doubt to the world that a chemical weapons attack took place given the thousands of sources," one said on the call.

The senior officials were authorised by the White House to speak on condition of anonymity. On Thursday White House deputy spokesman Josh Earnest had discouraged reporters from trusting anonymous administration sources, saying they should "place more credibility in on-the-record statements".

One of the officials said: "We feel like our case is strong, our case is clear: the Assad regime is responsible for this mass casualty chemical weapons attack."

Kerry's remarks came five days after he first signalled the US was planning to take tough action against Syria over its alleged use of chemical weapons in Damascus. Kerry and Obama have been involved in an intense round of diplomacy over the last week, seeking to conjure international backing for a tough response against Syria. However they are faced with launching a military assault with less support than George W Bush received for the 2003 war in Iraq.

Three out of the five permanent members of the UN security council, the only international authority that can sanction military action that is not in a nation state's self-defence, now oppose action.

Russian and Chinese opposition was widely expected. But the vote in the British parliament on Thursday came as a deep surprise to Washington, which appears to have taken for granted that London, which has spent months lobbying for tough action on Syria, would support strikes.

The White House has indicated it does not believe it needs the backing of Congress, nor the support of traditional allies, before taking action against Assad.

France is the only major power that has indicated it would support force against Syria. Hollande told Le Monde on Friday that France wants "proportional and firm action", adding that the chemical weapons attack in Syria "cannot and must not remain unpunished".

Germany has ruled out backing military action against Syria and it was not clear whether the US had significant support from the region, although the Arab League strongly condemned the Syrian regime.

Late on Thursday the administration held a conference call with congressional leaders and the chairs and ranking members of relevant committees. The White House said the call was to "to brief them on the administration's thinking and seek their input" on what to do about Syria.

Reports said senior administration officials assured members of Congress that there was "no doubt" Assad's forces were responsible for the chemical attack. Sixteen members of Congress asked questions during the 90-minute call; 11 apparently did not.

Administration officials, while pledging to work with Congress, were non-committal about whether a strike requires legislative approval, a longstanding tension between the congressional and executive branches of the US government.

There were few signs of a consensus emerging from the meeting. Democratic Senator Carl Levin, the chairman of the armed services committee, was notably more cautious than the administration's position. "I have previously called for the United States to work with our friends and allies to increase the military pressure on the Assad regime by providing lethal aid to vetted elements of the Syrian opposition," Levin said after the call.

"Tonight I suggested that we should do so while UN inspectors complete their work and while we seek international support for limited, targeted strikes in response to the Assad regime's large-scale use of chemical weapons against the Syrian people."

Eliot Engel of New York, the top Democrat on the House foreign affairs committee, said after the call that Obama was "still weighing his options and will continue to consult with Congress". Engel said he was persuaded that Assad's forces used chemical weapons "intentionally" against Syrian civilians on 21 August.

Politico reported that Nancy Pelosi, the top Democrat in Congress and a former House speaker, pressed Obama on the phone call to "do something" in response to the chemical attack.

Yet even some typically hawkish Republicans are balking at intervening in Syria. Jim Inhofe, the top Republican on the Senate armed services committee, issued a statement ahead of the call rejecting a Syria strike, partly on the grounds that Obama's Pentagon budget cannot afford it, and questioning the utility of a limited attack.

"It is vital we avoid shortsighted military action that would have little impact on the long-term trajectory of the conflict," Inhofe said. "We can't simply launch a few missiles and hope for the best."

More than 200 members of Congress, mostly Republicans, have signed a letter rejecting military action without the explicit permission of Congress.