Putin on Thursday allowed world leaders to thrash out their bitter differences on the Syria crisis at the G20 summit over dinner, as Obama pressed the case for military action in the face of strong Russian opposition.

Putin on Thursday allowed world leaders to thrash out their bitter differences on the Syria crisis at the G20 summit over dinner, as US President Barack Obama pressed the case for military action in the face of strong Russian opposition.

As tensions over the Syrian conflict threatened to torpedo the working schedule of the G20 summit outside Saint Petersburg, Putin made a last minute announcement for participants to air their views over dinner.

Russia has led opposition to US-led military action against Syria.

In New York, the US envoy to the United Nations accused Russia of holding the UN Security Council hostage while British Prime Minister David Cameron said London had fresh evidence of chemical weapons use.

In a new bid for a peaceful solution for Syria, the UN announced that its special envoy Lakhdar Brahimi was on his way to attend the summit to push for peace talks.

Russia meanwhile said Syria's Foreign Minister Walid Moallem would travel to Moscow on Monday as Obama seeks to convince US lawmakers to approve military action.

No information filtered out about the exchanges at the closed-door dinner hosted by Putin at the luxurious imperial palace of Peterhof outside Saint Petersburg.

Putin and Obama put on a show of smiles for the cameras as they shook hands just before the summit got under way on the shores of the Gulf of Finland.

Syria's allies remained unmoved by Obama's push, with Iran's supreme leader Imam Sayyed Ali Khamenei calling allegations of an August 21 chemical weapons attack by the regime a "pretext" to launch strikes against the country, and pledging to support Damascus "until the end".

According to US intelligence, more than 1,400 people living in rebel-held suburbs of Damascus were killed in the strike, which involved the use of sarin nerve gas.

Beyond convincing Russia, Obama has a tough sell ahead elsewhere, with China -- another veto-wielding Security Council member -- having already expressed its "grave concerns" over unilateral military strikes.

A political solution is the only way to end the Syria crisis, a senior Chinese official said on Thursday, warning world powers to be "highly prudent" over the issue.

"War cannot solve the problem in Syria," Chinese delegation spokesman Qin Gang told reporters at the G20.Pope Francis added his voice to the calls for a peaceful solution to the Syria crisis, warning against "futile pursuit of a military solution."

German Chancellor Angela Merkel has repeatedly ruled out her country's participation in any US-led military strike against Assad's regime, while the British parliament has also rejected the idea. 

EU president Herman van Rompuy said while the Damascus chemical attack was "a crime against humanity" there is "no military solution to the Syrian conflict".

UN-Arab League envoy Brahimi was in Saint Petersburg to help Secretary General Ban Ki-moon push at the G20 summit for an international peace conference on Syria first proposed by the United States and Russia in May.

"Providing more arms to either side is not the answer. There is no military solution," the UN quoted Ban as telling the G20 leaders.