SAINT PETERSBURG (AFP) - World leaders meet on Thursday at a G20 summit in Russia where US President Barack Obama will strive to bridge deep divisions over his push for military action against the Syrian regime's alleged use of chemical weapons.
Mr Obama cleared the first hurdle on Wednesday in his race to win domestic congressional backing for punitive strikes but is also seeking broader international support.
Speaking during a trip to Stockholm he said the world had set "a red line" for Syria and it could not now remain silent in the face of the alleged chemical weapons attack on Damascus suburbs.
But Russian President Vladimir Putin, a fierce opponent of the proposed military action, warned on the eve of the summit he is hosting in Saint Petersburg that it would be unacceptable for the West to go ahead with military action against Damascus without UN Security Council approval.
The Kremlin demanded "convincing" proof that the regime of Bashar al-Assad was responsible for using chemical weapons against its own people.
According to US intelligence, more than 1,400 people living in rebel-held suburbs of Damascus were killed in the strike, which involved the use of the sarin nerve gas.
Beyond convincing Russia, Mr Obama has a tough sell ahead elsewhere, with China - another veto-wielding Security Council member state - having already expressed its "grave concerns" over unilateral military strikes.
German Chancellor Angela Merkel has repeatedly ruled out her country's participation in any US-led military strike against Mr Assad's regime, while the British parliament has also rejected the idea.
But Mr Obama said in Sweden: "I didn't set a red line. The world set a red line," referring to international rules banning the use of chemical weapons, even in case of war.
"My credibility is not on the line," he said. "The international community's credibility is on the line and America and Congress's credibility is on the line."
The Syria conflict threatens to torpedo items on the G20 agenda - such as an "action plan" for sustainable and balanced global growth - even though it has not been formally pencilled in.
Emerging nations' appeal to developed nations to pursue an orderly exit from economic stimulus measures, thereby limiting damage to their markets, also risks being drowned out at the summit.
Syria is certain to be the top issue in the flurry of bilateral meetings between the leaders of the world's top 20 developed and emerging nations around the seaside Tsarist Konstantinovsky palace in Strelna on the Gulf of Finland seashore.
White House officials have said Mr Obama will hold meetings on the sidelines of the G20 with French President Francois Hollande, the main foreign backer of a strike on Syria, as well as the leaders of China and Japan.
While no formal bilateral meeting is planned with Mr Putin, a White House official suggested there would likely be some kind of dialogue.
Russian and US ties have sunk to a new low since the Cold War, over deep seated divisions over Syria, Russia's granting of asylum to US fugitive intelligence leaker Edward Snowden and a string of Russian laws targeting non-governmental organisations and opposition rallies.
In a fresh sign of the bilateral tensions John Boehner, the top Republican in the US House of Representatives, has rejected a request to meet a Russian delegation to discuss Syria, his spokesman said Wednesday.
Mr Putin, asked on Russian state television whether Russia would agree with US-led military strikes if it was proven that the Syrian regime had carried out the chemical attack, replied: "I do not exclude that." But he later told members of the board of human rights in the Kremlin that "only the UN Security Council can give approval for the use of force against another state".
The United Nations is making a desperate new push for a Syria peace conference even as the United States prepares a possible military strike, according to diplomats.
Talks on a conference are to be relaunched at the G20 summit, envoys said.
Syrian Deputy Foreign Minister Faisal Muqdad told AFP that his government was ready to retaliate in case of foreign military action.
"The Syrian government will not change position even if there is World War III. No Syrian can sacrifice the independence of his country," Mr Muqdad said.
"Syria has taken every measure to retaliate against... an aggression," he added.
Western military action against Syria had looked imminent last week, but Obama has deferred the move, and is seeking congressional backing in a vote scheduled for next Monday.
The Senate Foreign Relations Committee on Wednesday gave its backing by a 10-7 vote for the use of force.
Senate leaders said the full chamber will vote next week on the motion, when Mr Obama is expected to carry the day.
The amended resolution authorises military intervention with a 90-day deadline and bars US boots on the ground for combat purposes.
The House of Representatives will also begin its deliberations next week.
Secretary of State John Kerry said that Washington has recruited other nations to its cause, promising that allies in the region would support American and French strikes.
Since British lawmakers voted down a bid for strikes against the regime, Washington has found a firm partner in France.
With widespread scepticism in France over military action, the government in Paris has launched a major effort to bolster support.
French Prime Minister Jean-Marc Ayrault told lawmakers during a fiery parliamentary debate that "to not react would put peace and security in the entire region in danger." In Geneva, meanwhile, Antonio Guterres, the UN High Commissioner for Refugees, read a joint statement after talks with ministers Lebanon, Jordan, Turkey and Iraq calling for a political solution and an end to the fighting which has created millions of refugees.
Now in its third year, the popular uprising against the Assad regime has cost more than 100,000 lives.