By Andrew Woodcock, Press Association Political Editor<

 Tensions over Syria are expected to dominate a summit of the world's major

economies being held in Russia days before an expected US military assault on

the regime of President Bashar Assad, which host Vladimir Putin has warned would

be an act of aggression.<

 Prime Minister David Cameron risks being sidelined at the G20 gathering in St

Petersburg, after ruling out British involvement in any military action in the

wake of his shock Commons defeat on Syria last week.<

 He is not expected to have a formal bilateral meeting with US President Barack

Obama, who is leading the international drive for an armed response to Assad's

apparent breach of the prohibition on the use of chemical weapons.<

 Mr Obama said he was confident of receiving approval from Congress in a vote

expected next week 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.<

 Mr Obama cancelled a planned one-on-one meeting with Mr Putin in Moscow ahead

of the G20 gathering as it became clear the pair would not find common ground on


 Mr Cameron has said he will make the case at the G20 for a robust international

response to the use of chemical weapons.<

 And he promised to use Britain's "diplomatic muscle" to press the

international community to increase its support for an estimated two million

civilians forced to flee their homes during Syria's two-year civil war. Britain

has already contributed ‥348 million to the humanitarian relief effort and Mr

Cameron is expected to announce further aid in St Petersburg as he urges

fellow-leaders to dig deep for a UN appeal which has so far received less than

half of the sums requested.<

 He will also seek agreement from fellow-leaders on the need for better access

for humanitarian workers to those affected by the fighting - including the

establishment of safe routes for aid convoys and the removal of bureaucratic

obstacles to non-governmental agencies trying to enter Syria to provide help on

the ground.<

 The Prime Minister confirmed on Tuesday that the UK "can't be part and won't

be part" of any military strike as a result of his Commons defeat last week,

but warned that it would be "perilous" for the international community to let

Mr Assad escape unpunished.<

 He told MPs: "If no action is taken following President Obama's red line and

if no action is taken following this appalling use of chemical weapons, you have

to ask yourself what sort of Armageddon are the Syrian people going to be

facing?... I think we would see more chemical weapons attacks from the


 Syria does not even feature on the formal agenda for the summit of an

organisation which brings together 20 of the world's most important economies to

discuss issues like growth, financial stability and recovery from the global


 But the crisis is certain to dominate discussions on the margins of the two-day

meeting, which comes amid indications of healthier recovery in countries like

the UK, US and Japan, but faltering growth in emerging economies like China.<

 Britain will aim to use the formal proceedings in St Petersburg to press for

progress on priorities like tax transparency, reforms to the financial services

sector and free trade. The UK will resist an expected push by Brazil and

Argentina to water down anti-protectionist language in the summit communique.<

 Following phone calls in recent days with Canadian prime minister Stephen

Harper and European Commission president Jose Manuel Barroso, Mr Cameron will be

pressing for the completion of an EU-Canada free trade deal which eluded him at

the Lough Erne G8 summit hosted by the UK in Northern Ireland earlier this


 He will be joined by Chancellor George Osborne, as well as the Bank of

England's Mark Carney, who is the only central bank governor to attend in his

role as chair of the international Financial Stability Board.<