G20 leaders at Saint Petersburg summit remain divided over the Syrian conflict as they enter the final day of their meeting.
Italian PM Enrico Letta said the splits were confirmed during a working dinner in St Petersburg on Thursday.
A spokesman for the Russian presidency said a US strike on Syria would “drive another nail into the coffin of international law”.
At the UN, the US Ambassador Samantha Power accused Russia of holding the Security Council hostage by blocking resolutions.
Samantha Power 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 1,429 people in a poison-gas attack in the Damascus suburbs on August 21.
The UK says scientists at the Porton Down research laboratories have found traces of sarin gas on cloth and soil samples.
But Bashar al-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.
Samantha Power 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.
“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.”
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 Barack Obama and Russian President Vladimir Putin – discussed Syria over dinner on Thursday evening.
Enrico Letta said in a tweet that “the G20 has just now finished the dinner session, at which the divisions about Syria were confirmed”.
President Vladimir 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 the UK gave strong backing to President Barack Obama’s call for military action. The UK 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 correspondents in St Petersburg say opponents of US military intervention appear to far outnumber supporters within the G20.
However, the views of the G20 leaders on any US action could be the least of Barack Obama’s worries, as his real difficulties might lie back in the US.
He was nearly an hour late for Thursday’s G20 dinner. His aides said he had been trying to find time during the summit to call US members of Congress, who are due to vote next week on whether to back Barack Obama’s call for a military strike.
President Barack Obama also cancelled a trip to California on Monday in order to lobby Congress, as 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.
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”.
The Assad regime has been accused of using chemical weapons against Syrian civilians on several occasions during the 30-month conflict.
Some 100,000 people have died in the conflict, and more than two million Syrians are classified as refugees, according to the UN.