US Secretary of State John Kerry has arrived in the UK, as he continues a European tour aimed at increasing support for military action in Syria.
He will hold talks with Foreign Secretary William Hague in the wake of the vote by MPs against the action.
Mr Kerry, speaking in Paris, told a press conference before he left for London that 12 countries were now prepared to take military action against Syria. Those states would make their own announcements within 24 hours, he said.
He did not rule out returning to the UN Security Council to secure a Syria resolution once UN inspectors complete their report on the chemical weapons attack in Damascus that Washington says killed some 1,400 on August 21.
The French president, François Hollande, who is increasingly under pressure to seek a UN mandate before any military intervention in Syria, suggested that he could seek a resolution at the security council despite previous Russian and Chinese vetos.
Mr Kerry said: "On President Hollande's comments with respect to the UN, the president (Barack Obama), and all of us, are listening carefully to all of our friends. No decision has been made by the president."
It comes as Syrian President Bashar al Assad denied he was behind the chemical attack in a rare interview to US television network CBS.
CBS correspondent Charlie Rose, who interviewed Mr Assad in Damascus, said: "The most important thing, as he says, is that 'there's no evidence that I used chemical weapons against my own people'."
Meanwhile, Mohammad Jihad al Laham, the speaker of Syria's parliament, told Sky News that a US attack would result in the country retaliating with "all available force".
It comes as President Barack Obama prepared for a final push to try to convince US politicians to agree to take military action.
He will address his nation on Tuesday as he battles to secure the backing of Congress for strikes.
After meeting Arab leaders this morning, Mr Kerry said both the US and the Arab League were agreed that there was no military solution to the crisis in Syria but the Assad regime had crossed a "global red line".
He said: "What the United States is seeking to enforce is the standard with respect to the use of chemical weapons."
France has made no secret of its desire to play Washington's supporting partner after the US accused Syria of gassing its own people with sarin.
"We have more countries prepared to take military action than we actually could use in the kind of military action being contemplated," Mr Kerry said earlier.
Meanwhile, speaking on Sky News' Murnaghan programme former Defence Secretary Liam Fox said there was a case for another Commons vote "in the light of the wider evidence that is now available".
He said the debate had been about the consequences of intervention "but there wasn't enough debate, I don't feel, about the consequences of non-intervention."
"If we believe the regime in Syria still has chemical weapons and if, as many of us think, the opposition may also have access to chemical weapons and we send the signal that they can use them with impunity what does that mean for the safety ... of the ordinary people of Syria?"
Mr Kerry held talks with 28 EU foreign ministers in the Lithuanian capital Vilnius on Saturday before travelling to Paris. He is due to meet Arab leaders, including Palestinian President Mahmoud Abbas.
French foreign minister Laurent Fabius said there was agreement that all indications point to the Assad regime being responsible for a gas attack in Damascus that may have killed more than 1,400 people.
But in a joint statement member states stopped short of endorsing any US-led strike, stressing "the need to move forward with addressing the Syrian crisis through the UN process".
Later, Mr Kerry held further talks with Mr Fabius in Paris and spoke in French and English as he outlined the case for military action.
He said the world should not be "spectators to slaughter", but said President Obama had not yet decided whether to wait for the UN inspection report before taking action.
EU High Representative Catherine Ashton called for "a clear and strong response" to the attack but officials said the European Union has agreed that any potential strike against Syria should wait until after UN inspectors publish their report.
It is reported that the experts could submit their initial findings by the end of next week.