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, leaders 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.
Syria is not officially on the G20 agenda, so any discussion will be informal. Nonetheless, how the different countries line up will be illuminating and could have some bearing on how this crisis will play out.
Joining Russia in opposing US action will no doubt be China, given it too has consistently vetoed attempts to impose pressure on the Assad government at the UN Security Council and repeatedly insisted that any solution must be political. India and Indonesia’s views are less easy to identify.
Mr Obama knows he has the backing of French President Francois Hollande for military action, but – because of last week’s vote in the UK parliament – only diplomatic, but not military support from British Prime Minister David Cameron. Turkey has long advocated intervention in Syria, and Saudi Arabia is part of the Gulf coalition active in backing Syrian rebels.
Other Western allies at the table include Canada, Australia, South Korea and Japan, as well as Germany and EU leaders. But their separate views on the difficult question of whether or not to strike back against the Syrian military without UN approval are likely to be nuanced. Italy, also at the G20 table, has already voiced its objections.
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.
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.”
A new study of images apparently from the chemical attack on 21 August concludes that the rockets carrying the gas held up to 50 times more nerve agent than previously estimated, the New York Times reported.
The study was carried out by an expert in warhead design, Richard Lloyd, and Theodore Postol, a physicist at the Massachusetts Institute of Technology.
The German intelligence service, the BND, told German MPs in a confidential briefing on Wednesday that Syrian forces may have misjudged the mix of gases in the attack, the German magazine Der Spiegel reported.
This might explain why the death toll was much higher than in previous suspected attacks, the head of the BND was quoted as saying.
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 but fear the consequences of military retaliation.
US media reaction
“Mr Obama still hasn’t figured out after five years in office that America is the only enforcer of world order, and thus that there is no substitute for the president of the United States. Mr Obama can’t default to “the international community,” whatever that is, much less to Congress.” – Wall Street Journal
“A decade ago, I was aghast that so many liberals were backing the Iraq war. Today, I’m dismayed that so many liberals, disillusioned by Iraq, seem willing to let an average of 165 Syrians be killed daily rather than contemplate missile strikes that just might, at the margins, make a modest difference.” – Nicholas Kristof in New York Times
“The debate over Syria is a jumble of metaphors, proof that every discussion of military action involves an argument about the last war. Yet beneath the surface, the fight in Congress over President Obama’s proposed strike against Bashar al-Assad’s regime is a struggle to break free from earlier syndromes to set a new course.” – E.J Dionne Jr in Washington Post
Neither side is likely to get an easy ride, and finding a compromise 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.
Meanwhile, Syrian rebels have launched an assault on the religiously mixed village of Maaloula, held by government forces.
A Christian nun in Maaloula told the Associated Press news agency that the rebels had seized a mountaintop hotel and were shelling the community below.
The United Nations says more than 100,000 people have been killed since the uprising against President Assad began in March 2011.
More than two million Syrians are now registered as refugees, the UN says, with an additional 4.25 million displaced within the country, making it the worst refugee crisis since the 1994 genocide in Rwanda.