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 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 21 August.
The UK says scientists at the Porton Down research laboratories have found traces of sarin gas on cloth and soil samples.
But Mr Assad has blamed rebels for the attack, and China and Russia have refused to agree to a Security Council resolution against Syria.
The US and France are the only nations at the G20 summit to commit to using force in Syria. China and Russia insist any action without the UN would be illegal.
'Divisions confirmed'
Ms 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."
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.
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