• Meeting of the leading world economies is being held in St Petersburg this week
  
• President Putin warned that action against Syria without UN approval would be 'an aggression'
  
• President Obama said the credibility of the international community was on the line
• David Cameron has arrived - already facing suggestions he has been sidelined after ruling out British involvement
  
• Mr Obama cancelled a planned one-on-one meeting with Mr Putin in Moscow ahead of the G20 gathering
  

By Jill Reilly

PUBLISHED: 03:33 EST, 5 September 2013 | UPDATED: 05:47 EST, 5 September 2013

With tensions running high over Syria, the seating plan at today's G20 summit has reportedly been adjusted to put physical distance between host President Vladimir Putin and President Barack Obama.

Ahead of the meeting of the leading world economies in St Petersburg, Mr Putin warned that action without UN approval would be 'an aggression' as the relationship between the two countries reaches it lowest point since the Cold War.

But President Obama, who is leading the international drive for an armed response to President Bashar Assad's apparent breach of the prohibition on the use of chemical weapons, said the credibility of the international community was on the line. 

David Cameron arrived in Russia this morning and 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.

He is not expected to have a formal bilateral meeting with President Obama.

Mr Obama last night 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, 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.

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 Syria.

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 regime.'

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 downturn.

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 year.

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.