U.S. President Barack Obama defied pressure from fellow world leaders to abandon plans for air strikes against Syria, leaving deep divisions at a summit which overshadowed efforts on Friday to revive the global economy.
Leaders of the Group of Twenty (G20) developed and developing economies were expected to agree on a statement saying the world economy was not out of crisis but on the mend.
But Obama and Russian President Vladimir Putin remained far apart on Syria after a dinner discussion on the civil war in the Middle East which stretched deep into Thursday night.
"There has been a long discussion with a clear split in the group," a G20 source said of the dinner in a Tsarist-era palace in Russia's former imperial capital, St. Petersburg.
Japanese officials said there was an "exchange of frank opinions" on Syria.
Washington says troops loyal to Syrian President Bashar al-Assad carried out a poison gas attack which killed over 1,400 people in rebel-held suburbs of Damascus on August 21. But Moscow says there is no proof Assad's opponents were not responsible.
Unable to win United Nations Security Council backing for military action because of the opposition by veto-wielding Russia, Obama is seeking the backing of the U.S. Congress.
He stood firm in St. Petersburg, despite a warning by U.N. Secretary-General Ban Ki-moon about the need to find a political settlement to end the war: "Every day that we lose is a day when scores of innocent civilians die," Ban said.
Ben Rhodes, Obama's deputy national security adviser, told reporters the president had "once again underscored the very high confidence that we have" of Assad's role in the attack.
Obama also told the G20 leaders that it was important to uphold international norms against chemical weapons and depicted the Security Council as paralyzed.
Participants at the dinner said the tension between Putin and Obama was palpable but that they were polite to each other, and seemed at pains to avoid an escalation..
Obama also made his case on Syria at talks on Friday with Chinese President Xi Jinping, whose country opposes military action and also has veto powers on the Security Council.
Without referring to Syria, Obama said before meeting Xi: "Although there will continue to be some significant disagreements and sources of tension, I am confident that they can be managed."
He appeared isolated in St. Petersburg, despite France's support for military action, and the presence of allies such as Turkey and Saudi Arabia. But his actions suggested that winning the approval of Congress is his most important short-term goal.
Underscoring this, 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 Council over how to respond to the use of chemical weapons.
She said there was "no viable path forward in this Security Council" and accused Russia of holding it hostage.
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.