World leaders meeting for the final day of the G20 summit in Russia remain divided over military action in Syria.
Italian PM Enrico Letta said the splits in opinion were confirmed at Thursday's working dinner in St Petersburg.
A spokesman for the Russian presidency said a US military 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. 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 $3.3bn (£2bn) to deal with the Syrian refugee crisis up to the end of this year.
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.
But differences of opinion became obvious when world leaders - including Mr Obama and Russian President Vladimir Putin - discussed Syria over dinner on Thursday evening.
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 the UK gave strong backing to President 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.
And the BBC's Bridget Kendall says the views of the G20 leaders on any US action could be the least of Mr Obama's worries, as his real difficulties might lie back in the US.
President Obama 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 Mr Obama's call for a military strike.
Mr 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".
Aid for Syria
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.
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 Peskov.
Meanwhile, former Soviet leader Mikhail Gorbachev told the BBC that Mr Obama and Mr Putin should meet in order to find a solution to the Syrian conflict.
"They must strike up a conversation that will lead to the improvement of relations and stop the things which are happening now," he said.
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 two-and-a-half-year-old conflict, according to the UN.