US President Barack Obama is using his final day at the G20 summit to continue pushing for foreign support for a US military strike on Syria.
British sources say the leaders of France, Turkey, Canada and the UK gave Mr Obama strong backing.
But a Russian spokesman said a US strike would drive a “nail into the coffin of international law”.
The head of the UN, Ban Ki Moon, has also denounced talk of military action, calling it “ill-considered”.
Mr Ban said such attacks – responding to the apparent use of chemical weapons in Syria – could lead to more sectarian violence.
Meanwhile the UN says it needs another $3.3bn (£2bn) to deal with the Syrian refugee crisis up to the end of this year.
Mr Obama discussed the deepening crisis in Syria with other G20 delegates at a lengthy working dinner on Thursday night – and it was there, correspondents say, that their differences in opinion became obvious.
The BBC’s Bridget Kendall, who is at the St Petersburg summit, says that on the final day of their talks these divisions are even more entrenched.
The US government accuses Syrian President Bashar al-Assad’s forces of killing 1,429 people in a poison-gas attack in the suburbs of Damascus on 21 August.
But Mr Assad has blamed rebels for the attack – and the Russian President, Vladimir Putin, says it still is not clear which side used the weapons.
While the UK, Canada and Turkey all support Mr Obama’s call for action, the only leaders at the G20 meeting to commit to force in Syria are the US and France.
Correspondents in St Petersburg say opponents of US military intervention appear to far outnumber supporters within the G20.
China and Russia, which have refused to agree to a Security Council resolution against Syria, insist any action without the UN would be illegal.
Aid for Syria
The UN is appealing for more aid for people in Syria, and also for the estimated two million Syrians who have fled their country.
The UN humanitarian chief, Valerie Amos, told the BBC that donor countries should “look again” at their contributions and be “as generous as they can”.
UK Prime Minister David Cameron announced on Friday that the UK would give an additional £52m ($80m) in aid for Syria – much of which will go towards medical training and equipment to help civilians targeted by chemical attacks.
Some analysts say Mr Cameron has been sidelined at the summit because the UK parliament has already voted against military strikes.
A senior Russian aide reportedly dismissed Britain as “just a small island: no-one pays any attention to them” – comments later denied by Mr Putin’s chief spokesman Dmitry Peskov.
Our correspondent Bridget Kendall says the views of the G20 leaders could be the least of Mr Obama’s worries, as his real difficulties might lie back in the US, where Congress is preparing to vote next week on whether to back military strikes.
A poll commissioned by the BBC and ABC News suggested more than one-third of Congress members were undecided whether or not to back military action – and a majority of those who had made a decision said they would vote against the president.
Syria’s parliamentary speaker has written to the speaker of the House of Representatives urging members not to rush into an “irresponsible, reckless action”.
Meanwhile the US ambassador to the UN, Samantha Power, has accused Russia of holding the Security Council hostage by blocking resolutions.
Ms Power said the council was no longer a “viable path” for holding Syria accountable for war crimes.
She told a news conference in New York: “Even in the wake of the flagrant shattering of the international norm against chemical weapons use, Russia continues to hold the council hostage and shirk its international responsibilities.