The US and Russia are striving to garner support for their respective strategies over the Syrian crisis at the G20 summit in St Petersburg.

While Washington is bent on carrying out a military strike against President Bashar al-Assad's regime, Moscow insists any intervention without the UN mandate would amount to an act of "aggression" by the US.

The sharp differences among G20 leaders came to the fore at the summit. Italian Prime Minister Enrico Letta said the G20 nations remain divided following the first day of the talks.

"The G20 has just now finished the dinner session at which the divisions about Syria were confirmed," wrote Letta on his Twitter account.

Russian President Vladimir Putin's spokesperson Dmitry Peskov said: "A number of states maintained the stance on the necessity of hasty measures ignoring any legitimate international institutions."

The summit is usually a platform for world economic debate, but it has been dominated by the Syrian crisis.

Moscow, a key ally of Assad, and China have been saying the US needs more credible evidence about the alleged chemical attack before taking military action against Syria. The US and France have been trying to gather support for the strike.

While the British parliament ruled out its participation in the proposed action, German Chancellor Angela Merkel has said her country would also hold off.

At the UN headquarters in New York, US ambassador Samantha Power accused Russia of holding the UN hostage over the Syrian crisis.

"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," said Power in a clear indication that the White House may not wait for UN approval for the strike.

UN Secretary General Ban Ki-moon, who is at the G20 summit in Russia, reiterated that the Obama administration needs the international body's mandate for an intervention.

Stressing the gravity of the situation, he said: "Let us remember: every day that we lose is a day when scores of innocent civilians die. There is no military solution."