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. At the UN, US ambassador Samantha Power accused Russia of holding the Security Council hostage by repeatedly blocking resolutions. 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 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. But differences of opinion were confirmed 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. President Obama said last week the US military was prepared to launch a "limited, narrow" strike on targets in Syria, but promised to allow Congress a vote on the issue. The BBC's Bridget Kendall in St Petersburg 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 earlier said he had been trying to find time during the summit to call US members of Congress. Mr Obama also cancelled a trip to California on Monday 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". 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 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.