President Vladimir 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 the Syrian President Bashar al-Assad over a chemical attack on August 21 outside Damascus that Washington says was perpetrated by the regime.

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 Muallem 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 Ayatollah 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.

"We have just been looking at some samples taken from Damascus in the Porton Down laboratory in Britain which further shows the use of chemical weapons in that Damascus suburb," Cameron told BBC TV from the G20 summit.

A British source told AFP that a soil and cloth sample taken from the site of the alleged chemical attack in the Syrian capital "has tested positive for sarin".

--- 'War cannot solve the problem' ---

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.

The US-Russia push for peace talks has languished as ties between the two world powers sank to a new post-Cold War low over deep divisions on Syria, Russia's granting of asylum to US intelligence leaker Edward Snowden and a string of hardline Russian laws.

While no formal bilateral sit-down meeting is planned between Obama and Putin, a White House official suggested there would likely be some kind of more informal conversation.

With Russia expected to use its UN Security Council veto to block any military action, US ambassador to the United Nations Samantha Power on Thursday accused Russia of holding the UN Security Council "hostage" over the Syria crisis.

Western military action against Syria had looked imminent last week, but Obama deferred the move and is seeking backing from Congress when it resumes sitting next week.

The Senate Foreign Relations Committee on Wednesday gave its backing by a 10-7 vote for the use of force. Senate leaders said the full chamber will vote next week on the motion, when Obama is expected to carry the day.

The amended resolution authorises military intervention with a 90-day deadline and bars US boots on the ground for combat purposes.

The House of Representatives will also begin its deliberations next week.

Since British lawmakers voted down a bid for strikes against the regime, Washington has found a firm partner in France.

Now in its third year, the popular uprising against the Assad regime has cost more than 100,000 lives.