US secretary of state John Kerry will continue his quest to secure support for military action against Syria in talks with Foreign Secretary William Hague.
Mr Kerry is on a whirlwind diplomatic push in Europe as Barack Obama prepares to mount a concentrated attempt to persuade a sceptical Congress to back him.
The president will take to the airwaves ahead of a nationally televised address on Tuesday, the eve of the first Senate vote on a "limited and specific" intervention by US forces.
Surveys suggest he faces a steep uphill battle to win Congressional approval for any armed response to the deadly August 21 chemical weapons attack on the outskirts of Damascus.
Mr Kerry flew into London from Paris where he claimed the support of "a number" of Arab League nations for a "strong international response" following talks in the French capital.
"All of us agreed, with not one dissenter, that Assad's deplorable use of chemical weapons, which we know killed hundreds of innocent people, including at least 426 children, on this occasion, this one occasion, crosses an international, a global red line," he said. "What the United States is seeking - not alone but with others, an increasing number - is to enforce the standard with respect to the use of chemical weapons. We are not seeking to become engaged in or party to or take over Syria's civil war."
European Union foreign ministers at the weekend also backed the need for a "clear and strong response" to the use of chemical weapons after hearing from him. 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".
French president Francois Hollande, the most vocal supporter of military action outside Washington, has said he will await the findings of UN weapons inspectors before deciding whether to act. Mr Kerry opened his Paris press conference in French as he kept up efforts to woo the White House's principal ally following the shock Commons vote ruling out any UK involvement.
Mr Hague admitted the Americans were "disappointed" by David Cameron's failure to secure the backing of MPs but remained "very committed to what we call the special relationship". The Prime Minister has ruled out bringing back the issue to Parliament but faces pressure from some quarters to do so if there is a significant change in the circumstances.
Mr Hague said it would be "alarming" if the UK vote was repeated in the US and elsewhere. The risks of the world failing to respond "are greater than the risks of doing so in a limited, proportionate and careful way", he told the BBC. "If it is decided in the various parliaments of the world that no one will stand up to the use of chemical weapons and take any action about that, that will be a very alarming moment in the affairs of the world."