World leaders from the G20 group of nations are set to meet in Russia amid sharp differences over military action against Syria's government.
Ahead of the talks, Russia's Vladimir Putin warned that action without UN approval would be "an aggression".
But President Barack Obama said the credibility of the international community was on the line.
While Syria is not officially on the G20 agenda, ministers are expected to discuss it on the sidelines.
The annual summit of the G20 group of developed and developing nations which opens in St Petersburg is supposed to concentrate on the global economy.
On the eve of the summit, a US Senate panel approved the use of military force in Syria, in response to an alleged chemical weapons attack.
The proposal, which now goes to a full Senate vote next week, allows the use of force in Syria for 60 days with the possibility to extend it for 30 days.
The measure must also be approved by the US House of Representatives.
No easy ride
The Damascus government is accused of using chemical weapons against civilians on several occasions during the 30-month conflict - most recently on a large scale in an attack on 21 August on the outskirts of the capital.
The government of Syrian President Bashar al-Assad has denied involvement and said the rebels were responsible.
The US has put the death toll from that incident at 1,429 - though other countries and groups have given lower figures - and says all the evidence implicates government forces.
The Russian president said it was "ludicrous'' that Syrian President Bashar al-Assad, an ally of Russia, would use chemical weapons at a time when it was gaining ground against the rebels.
"If there is evidence that chemical weapons were used, and by the regular army... then this evidence must be presented to the UN Security Council. And it must be convincing," Mr Putin said in an interview on Wednesday.
But he added that Russia would "be ready to act in the most decisive and serious way" if there was clear proof of what weapons were used and who used them.
For his part, Mr Obama is trying to build support in the US for punitive military action against the Syrian government.
Speaking in Sweden before going on to Russia, he said the world should stick to its own "red line" against the use of chemical weapons.
"The international community's credibility is on the line," Mr Obama said. "America and Congress's credibility is on the line, because we give lip service to the notion that these international norms are important."
BBC diplomatic correspondent Bridget Kendall, who is in St Petersburg, says that while both leaders have allies at the G20, the battle will be for the middle ground - those countries who share concern about chemical weapons having been used but fear the consequences of military retaliation.
Neither side is likely to get an easy ride, and finding a compromise to narrow the gap looks highly unlikely, our correspondent adds.
France has strongly backed the US plan for military action, and the French parliament debated the issue on Wednesday, although no vote was due to be held.
Source: BBC
