David Cameron has arrived in Russia for a summit of the world's major economies that is set to be dominated by tensions over Syria.
The Prime Minister is already facing suggestions that he has been sidelined after ruling out British involvement in any military action in the wake of his shock Commons defeat last week.
It has emerged that Mr Cameron is not expected to hold a formal bilateral meeting with US president Barack Obama, who is leading the international drive for armed reprisals to President Bashar Assad's apparent chemical weapons use despite opposition from Russian counterpart Vladimir Putin.
Mr Obama has cleared the first hurdle to obtaining Congressional approval for a strike, as the influential Senate Foreign Affairs Committee backed the use of force by a margin of 10-7 on Wednesday night, moving the measure to a full Senate vote next week. The proposal allows the use of force for 60 days, with the possibility of a 30-day extension.
The president has said he is confident of receiving approval from Congress for "limited and proportionate" military action, which he said would not involve US troops putting "boots on the ground" in Syria.
Mr Assad had flouted a chemical weapons ban enshrined in treaties signed by governments representing 98% of the world's population, he said, adding: "I didn't set a red line. The world set a red line."
Speaking in Sweden as he travelled to St Petersburg, the US president said the credibility of the international community was "on the line" if it allowed Assad to act with impunity.
He repeated his "high confidence" that the regime was to blame for the sarin gas attack on a suburb of Damascus on August 21 which the White House believes killed more than 1,400 people, including 400 children.
Meanwhile, France's prime minister Jean-Marc Ayrault, rallying support for French involvement in military action, told the National Assembly in Paris there was no doubt the Syrian government was to blame and a failure to react would allow Mr Assad to launch a similar attack again. But Mr Putin said any evidence on culpability for the attack should be presented to the UN Security Council - where Russia has repeatedly blocked reprisals against Mr Assad.
The Russian president, a long-time ally of the Syrian dictator, said he "doesn't exclude" Moscow voting in favour of a military response if the Security Council is provided with "evidence that would be obvious and prove beyond doubt who did it". But he said that a US strike in the absence of Security Council approval would amount to aggression.