G20 leaders remain divided over the Syrian conflict as they enter the final day of their Russian summit.
Italian PM Enrico Letta said the splits were confirmed during a working dinner in St Petersburg on Thursday.
A spokesman for Russian President Vladimir Putin said an American strike on Syria would "drive another nail into the coffin of international law".
At the United Nations, United States Ambassador Samantha Power accused Russia of holding the Security Council hostage by blocking resolutions, the BBC reports.
She said the Security Council was no longer a "viable path" for holding Syria accountable for war crimes.
The US government accuses President Bashar al-Assad's forces of killing 1429 people in a poison-gas attack in the Damascus suburbs on 21 August, and scientists at Britain's Porton Down research laboratories have found traces of sarin gas on cloth and soil samples.
But Mr Assad has blamed rebels for the attack. China and Russia, which have refused to agree to a Security Council resolution against Syria, insist any action without the UN would be illegal.
The US and France are the only nations at the G20 summit to commit to using force in Syria.
The United Nations says it needs another $US3.3 billion to deal with the Syrian refugee crisis up to the end of this year.
"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," Ms Power told a news conference in New York.
"What we have learned, what the Syrian people have learned, is that the Security Council the world needs to deal with this crisis is not the Security Council we have."
Obama seeking support
US President Barack Obama is thought to be trying at the G20 summit to build an international coalition to back strikes against military targets in Syria.
But differences of opinion became obvious when world leaders - including Mr Obama and Mr Putin - discussed Syria over dinner on Thursday.
The Italian prime minister said in a tweet that "the G20 has just now finished the dinner session, at which the divisions about Syria were confirmed".
President Putin's press spokesman, Dmitry Peskov, said after the dinner that the G20 was split down the middle, with some countries seeking hasty action and others wanting the US to go through the UN Security Council.
British sources say the leaders of France, Turkey, Canada and Britain gave strong backing to President Obama's call for military action. British Chancellor of the Exchequer George Osborne said the Turks put a "very strong argument about how the world must respond to the use of chemical weapons".
But BBC correspondents in St Petersburg say opponents of US military intervention appear to far outnumber supporters within the G20.
The United Nations is appealing for more aid for the estimated two million Syrians who have fled their country. Another 4.25 million are internally displaced.
UN humanitarian chief Valerie Amos told the BBC that donor countries should "look again" at their contributions and be "as generous as they can".
British Prime Minister David Cameron announced on Friday Britain would give an additional £52m in aid for Syria - much of which will go towards medical training and equipment to help civilians targeted by chemical attacks.
The Assad regime has been accused of using chemical weapons against Syrian civilians on several occasions during the 30-month conflict.
An estimated 100,000 people have died in the two-and-a-half-year-old conflict, according to the UN.