The United States was on a clear path to military intervention in Syria tonight, declaring in the most robust terms yet that it now has “high confidence” that the regime of President Bashar al-Assad ordered gas attacks on a suburb of Damascus last week and asserting the assault killed no fewer than 1,429 people.

Speaking from the White House, a sober President Barack Obama said Syria’s actions represented a “challenge to the world” and to American national security. He added that he had not made a final decision and was considering only a “limited, narrow act”. He emphasized: “We’re not considering any open ended commitment. We’re not considering any boots on the ground approach.” He added, however, that the US has an obligation “as a leader in the world” to hold countries accountable if they violate “international norms”.

At the State Department, the Secretary of State, John Kerry, argued in more passionate language that the Syrian regime had committed a “crime against humanity” that could not go unpunished. “History will judge us extraordinarily harshly if we turn a blind eye,” he said, adding that there were 426 children among the dead. “This is the indiscriminate, inconceivable horror of chemical weapons. This is what Assad did to his own people.”

Download the new Independent Premium app Sharing the full story, not just the headlines

Echoing the contents of a four-page intelligence assessment simultaneously made public, Mr Kerry ran through a catalogue of what he said are now known facts. He said the US had evidence that regime personnel were on the ground in the targeted area three days before, “making preparations”. He said the US knows where the rockets armed with chemical agents were launched from and at what time. He also said that “Syria regime elements were told to prepare for the attack by putting on gas masks and taking precautions associated with chemical weapons”. He added: “We know these were specific instructions.”

In his presentation, Mr Kerry also cited the Arab League, Turkey, Australia and “our oldest ally, the French” as among those in the world expressing support for punishment against the regime. He notably omitted all mention of Britain, where a previous promise by the Prime Minister, David Cameron, to join military action was effectively nullified by his defeat in Parliament on Thursday.

Adding that they have a direct intercept of a regime official acknowledging that chemical weapons had been used and expressing concern the world would find out, Mr Kerry said “the American intelligence community has high confidence” in its findings. “This is common sense, this is evidence. These are facts,” he said.

Tonight Syria issued a statement accusing Mr Kerry of a “desperate attempt” to justify a military strike. The Syrian Foreign Ministry added that claims the regime had used chemical weapons were “lies and baseless”.

Earlier, Chuck Hagel, the US Defence Secretary, similarly left little doubt that the mission would move forward with or without Britain. “I don’t know of any responsible government around the world... that has not spoken out in violent opposition to the use of chemical weapons on innocent people,” Mr Hagel said during an official visit to the Philippines.

There was little sign, however, that the White House would tolerate much more delay. The UN inspectors left Syria early this morning, as planned, which removes one obstacle to launching military strikes. On Tuesday Mr Obama leaves for Sweden and thereafter Russia for a G20 summit; he might prefer to be at home in the Oval Office to launch the operation. There was speculation tonight that military action could be launched within days.

Mr Cameron may yet be playing a part in the planning of the strikes even if the British military, as things stand now, may not participate. The White House confirmed that President Obama tonight placed phone calls to two foreign leaders to further discuss his intentions - President Hollande of France and the Prime Minister. In a statement, moreover, it seemed at pains to play down any perceived damage to the ties with Britain in the wake of Thursday night’s vote in Parliament. “As always, the United States values the special relationship with the United Kingdom, a close ally and friend. The President and Prime Minister agreed to continue to consult closely on Syria and the broad range of security challenges that our two countries face together,” it said.

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

That France was giving credence to the notion of international backing for military action was ironic given Paris’s opposition to the Iraq war in 2003, which led Congress to ditch “French fries” from its menus in favour of “Freedom fries”. Since France has deeper ties with Syria, which it once ruled, than Iraq, the change is particularly dramatic.

“The chemical massacre of Damascus cannot and must not remain unpunished,” President Hollande said in an interview with Le Monde today. “I won’t talk of war, but of a sanction for a monstrous violation of the human person,” he said. “It will have a dissuasive value.” Mr Hollande said he might approve a mission even before a debate that is scheduled in the parliament for Wednesday. “If I have [already] committed France, the government will inform [politicians] of the means and objectives,” he said.

Mr Obama acknowledged America was “war weary”. A poll by NBC said half of Americans opposed military action against Syria, with only 42 per cent supporting it. Moreover four in five Americans said Mr Obama should seek the approval of Congress first. Mr Kerry also noted public reluctance, but said: “Fatigue does not absolve us of our responsibility. Just longing for peace does not necessarily bring it about.”

Nor are Mr Obama’s predecessors of one voice. The former President George W Bush expressed sympathy with Mr Obama’s position. “The President has a tough choice to make, and if he decides to use our military, he’ll have the greatest military ever backing him up,” he told Fox News. Elsewhere, Jimmy Carter urged the White House to seek UN backing. “A punitive military response without a UN mandate or broad support from Nato and the Arab League would be illegal under international law and unlikely to alter the course of the war,” he said.

Reaction to a conference-call briefing on the contents of the Syrian intelligence dossier with leading members of Congress on Thursday night appeared mixed and apparently did little to quell calls for Mr Obama to consult with Congress before taking action. “There needs to be more consultation with all Members of Congress,” House Minority Leader Nancy Pelosi said in a statement. “And that the case needs to be made to the American people.”

Kerry fails to mention Britain in list of nations backing US

The Foreign Secretary, William Hague, needs more than one hand to count the number of times he has stood in that same spot in the ornate Treaty Room of the State Department where John Kerry made his Syria presentation. He might almost have had his own keys given the sanctity of the “special relationship”.

He can give them back now. How jarring it was to hear Kerry not just omit all reference to Britain today as he went down the list of countries and organisations that have voiced support for America’s stand on Syria but then to give special standing to “our oldest ally” the French.

The dig appeared was especially acid if you know its historical context. Kerry was making reference to France coming to the side of America against Britain in the revolutionary war that began in 1776.

“America should feel confident and gratified that we are not alone in our condemnation and we are not alone in our will to do something about it and to act,” Mr Kerry declared. Gratified, however, is clearly not the first sentiment that Washington is feeling today when it comes to Britain - not its oldest ally.

Activists died filming attacks

Local activists whose videos of the chemical attack in a Damascus suburb this month prompted Western politicians to rethink their stance on the Syrian conflict paid the ultimate price for their efforts, dying after inhaling the gases, according to a new report.

Razan Zaitouneh of the Violations Documentation Center (VDC) in Syria told Foreign Policy magazine that a VDC team and media staff from a local coordination committee rushed to the suburb of Zamalka to document the attack soon after it was reported on 21 August. She said only one of those activists survived the attack.

“Chemical attacks, on the first day of the massacre, claimed the lives of many media activists in Zamalka coordination because they inhaled the chemical toxic gases,” the sole surviving activist, Murad Abu Bilal, told Ms Zaitouneh in a YouTube interview, according to Foreign Policy. “They went out to shoot and collect information about the chemical attack, but none of them came back.”