Obama refused to blink after Russian President Vladimir Putin led a campaign to talk him out of military intervention at a two-day summit of the Group of Twenty developed and developing economies in St. Petersburg.
He persuaded nine other G20 nations plus Spain to join the United States in signing a statement calling for a strong international response, although it fell short of supporting military strikes, underscoring the deep disagreements that dominated the summit.
A senior U.S. official said German Chancellor Angela Merkel, the only European leader at the summit who did not sign the statement, held off because she wanted to let the European Union have a chance to weigh in first.
Leaders of the G20, which accounts for 90 percent of the world economy and two-thirds of its population, put aside their differences to unite behind a call for growth and jobs and agreed the global economy was on the mend but not out of crisis.
But there was no joint statement on Syria, despite a 20-minute one-on-one talk between Obama and Putin on the sidelines of the summit on Friday, following a tense group discussion on the civil war over dinner late on Thursday.
"We hear one another, and understand the arguments but we don't agree. I don't agree with his arguments, he doesn't agree with mine," Putin told a closing news conference dominated by questions about Syria.
Participants at the dinner said the tension between Putin and Obama was palpable but that they seemed at pains to avoid an escalation. Obama said credit was due to Putin for facilitating the long discussion of the Syrian crisis on Thursday night.
But he defended his call for a military response to what Washington says was a chemical weapons attack by forces loyal to Syrian President Bashar al-Assad that killed more than 1,400 people in rebel-held suburbs of Damascus on August 21.
"Failing to respond to this breach of this international norm would send a signal to rogue nations, authoritarian regimes and terrorist organizations, that they can develop and use weapons of mass destruction and not pay a consequence. And that's not the world that we want to live in," Obama told a separate news conference..
Putin said Washington had not provided convincing proof that Assad's troops carried out the attack and called it a "provocation" by rebel forces hoping to encourage a military response by the United States.
Chinese President Xi Jinping tried to dissuade Obama from military action during talks on Friday, telling him that Beijing expected countries to think twice before acting. U.N. Secretary-General Ban Ki-moon warned against military action that did not have the approval of the U.N. Security Council.
Unable to win Security Council backing because of the opposition by veto-wielding Russia and China, Obama is seeking the support of the U.S. Congress instead.
He declined to speculate whether he would go ahead with a military strike in Syria if Congress opposed it, but said most G20 leaders condemned the use of chemical weapons even if they disagreed whether to use force without going through the United Nations.
"The majority of the room is comfortable with our conclusion that Assad, the Assad government, was responsible for their use," he said.
Those who signed up to the call for a strong international response were the leaders or other representatives of Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey, Britain and the United States.
The senior U.S. official said the statement had been worked out over the past two days and while there were changes to a draft produced by U.S. national security adviser Susan Rice, the final version had in it everything the United States wanted. A final chat at the summit between Obama and French President Francois Hollande sealed the deal, the official said.
The statement's endorsement of the U.S. approach represents an implicit backing of the use of military power, even if that is not spelled out in the statement, the official added, contradicting Putin's assertion that the only countries to support the use of force are Canada, Saudi Arabia, France and Turkey.
Washington's ambassador to the United Nations, Samantha Power, made clear on Thursday that the United States had given up trying to work with the Security Council on the issue, and accused Russia of holding it hostage.
Hollande, who supports Obama over military action against Syria, said he would try to bring together a coalition of states in favor of such an intervention if the Security Council could not agree on action.
HUMAN RIGHTS TALKS
The dispute over Syria has deepened strains in U.S.-Russian ties, already difficult because of differences over human rights and Moscow's hosting of Edward Snowden, a spy agency contractor who revealed details of U.S. surveillance programs. Putin said Obama had not requested Snowden's extradition on Friday, adding that it would be impossible anyway.
Obama later met rights activists, including gay rights campaigners, to show support for civil society in Russia, where critics say Putin has clamped down on dissent in his third term.
But some invitees declined to attend, citing what they said were repeated changes in the timing of the meeting. One added her voice to warnings against a military strike on Syria.
The G20 achieved unprecedented cooperation between developed and emerging nations to stave off economic collapse during the 2009 financial crisis, but the harmony has since waned.
Despite their differences, the leaders agreed on a summit declaration that the global economy is improving although it is too early to declare an end to crisis.
The leaders stuck closely to a statement issued by G20 finance ministers in July that demanded monetary policy changes must be "carefully calibrated and clearly communicated".
"Our most urgent need is to increase the momentum of the global recovery, generate higher growth and better jobs, while strengthening the foundations for long-term growth and avoiding policies that could cause the recovery to falter or promote growth at other countries' expense," the leaders said.
Member states are at odds as the U.S. recovery gains pace, Europe lags, and developing economies worry about the impact of the Federal Reserve's plans to stop a bond-buying program that has helped stimulate the U.S. economy.
The BRICS emerging economies - Russia, China, India, South Africa and Brazil - have agreed to commit $100 billion to a currency reserve pool that could help defend against a balance of payments crisis, but the mechanism will take time to set up.