The crisis in Syria is expected to dominate the final day of the G20 summit in Russia as international tensions continue over how to deal with the conflict.

Relations among world leaders have become heated over possible US military action against President Bashar al Assad's regime, following a deadly chemical weapons attack on August 21.

The US and Russia have been unable to agree on a way forward and President Barack Obama said he had "hit a wall" with his counterpart Vladimir Putin, who has warned the use of force without UN approval would be "aggression" and a violation of international law.

Russia, a staunch ally of Syria, also reportedly dismissed Britain as a "small island no one pays any attention to" as relations boiled over at the summit in St Petersburg.

The alleged comments, thought to include a reference to Soviet oligarchs "buying Chelsea" and attributed to President Putin's spokesman Dmitry Peskov, were later denied by Russian officials.

US Ambassador Samantha Power told a news conference in New York that Russia has held the UN Security Council "hostage" over Syria, and Mr Obama's administration did not expect that to end:

She said, "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," she said.

"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."

Cameron works to remain involved

Earlier, British Prime Minister David Cameron said he would push world leaders at the summit to agree more aid for the estimated two million Syrian refugees left homeless by the civil war.

"Every 15 seconds there is another Syrian refugee" he said, "Millions of people are homeless and they need our help."

The summit was tasked with fostering strong, sustainable growth through encouraging long-term investment among nations, but has been overshadowed by the Syria crisis.

Scientists in the UK found further evidence on Thursday that deadly sarin gas was used in the Damascus attack.

But the discovery, alongside evidence from France and the US, is unlikely to cause Russia to change its position.

On Thursday, the Syrian government wrote to the US Congress urging its members not to support the use of military force.

The letter urged Congress to "communicate with us through civilised dialogue rather than the language of fire and blood".