World leaders are set to meet at a G20 summit in Russia where American 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.
With pressure mounting on the G20 to make concrete progress towards ending the conflict, the United Nations announced that its special envoy Lakhdar Brahimi was on his way to the summit, in St Petersburg, to push for peace.
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, AFP reports.
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 St 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 1400 people living in rebel-held suburbs of Damascus were killed in the strike, which involved the use of the sarin nerve gas.
Tough sell for Obama
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.
In St Petersburg, Vice-Minister of Finance Zhu Guangyao emphasised that "China believes that only a political solution ... is the way to solve the Syria problem," and warned of a negative impact on the world economy in case of military action, reports AFP.
German Chancellor Angela Merkel has repeatedly ruled out her country's participation in any US-led military strike against 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.
The Syria conflict has still not been formally pencilled into the agenda of the G20 summit, which begins on Thursday on the shores of the Gulf of Finland at a former imperial palace outside St Petersburg.
But discussions about the Syria crisis still threaten to completely overshadow leaders' efforts to promote a crucial economic agenda of stimulating growth and cracking down on tax avoidance.
The United Nations is making a desperate new bid for a Syria peace conference, according to diplomats.
"It is time for the parties to stop fighting and start talking. The Syrian people need peace," Mr Ban said in a lecture at St Petersburg State University on Wednesday.
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 sit-down meeting is planned with Mr Putin, a White House official suggested there is likely be some kind of informal conversation.