World leaders are locked in a divisive debate over Syria, at the end of the first day of the G20 summit in Russia.
Opening the summit, Russian President Vladimir Putin confirmed Syria would be discussed at the working dinner.
US President Barack Obama is pushing for support for military strikes. Russia and China have warned the US not to take action without UN backing.
Meanwhile, Syria's parliamentary speaker has written to the speaker of the US House of Representatives.
In his letter to John Boehner, Mohammed Jihad al-Lahham urged the US Congress not to rush into what he called "irresponsible, reckless action".
Mr Putin, in his opening remarks, told the leaders gathered in St Petersburg that some participants had asked for time to discuss "very acute topics of international politics, in particular the situation around Syria", even though it is not on the agenda.
"I suggest we do this during dinner so that we... in the first part can discuss the [economic] problems we had gathered here for," he said.
The BBC's Bridget Kendall in St Petersburg says, in the short-term, the leaders can only hope that their discussion over Syria at dinner does not descend into an even more bitter row than it is already.
The government of Syrian President Bashar al-Assad is accused of using chemical weapons against civilians on several occasions during the 30-month conflict.
The US said the most recent attack took place on 21 August on the outskirts of Damascus and had killed 1,429 people. It released an intelligence assessment blaming the Syrian government.
German and US media reports published further analysis on Thursday both claiming that the gas used in the attack was more potent than expected, and that Syrian forces may have got the mix wrong.
UK Prime Minister David Cameron has told the BBC British scientists have uncovered further evidence linking the Damascus attack to the use of chemical weapons.
Mr Assad's government has denied involvement and said the rebels were responsible.
Some 100,000 people have died in the conflict, and more than two million Syrians are classified as refugees, according to the UN.
On the G20 sidelines, Mr Putin's spokesman once again dismissed the US intelligence assessment on the Damascus attack.
"We can't accept proof that is a long way from being convincing," said Dmitry Peskov.
Mr Obama, who is trying to build support in the US for military action against the Syrian government, held informal talks with Japanese Prime Minister Shinzo Abe over the issue.
Mr Obama said Japan and the US had a "joint recognition" that the use of chemical weapons in Syria was a tragedy and a violation of international law.
Mr Abe has not stated publicly whether he supports military strikes.
In the run-up to the summit, the US and Russia have engaged in tit-for-tat insults.
US Secretary of State John Kerry accused Russia of "obstructionism", and Mr Putin responded by calling Mr Kerry a "liar".
On Thursday, the US envoy to the UN continued the tough rhetoric, accusing Russia of holding the UN Security Council hostage.
Samantha Power said that Russia, which has twice blocked resolutions on Syria, had ensured that there was "no viable path" for action through the Security Council.
Former Soviet leader Mikhail Gorbachev told the BBC's Newsnight programme in an interview to be broadcast later on Thursday that Mr Obama and Mr Putin must meet.
"They must strike up a conversation that will lead to the improvement of relations and stop the things which are happening now," he said.
In other developments:
- Pope Francis has written a letter to Mr Putin urging world leaders to "lay aside the futile pursuit of a military solution" in Syria
- Chinese Vice Finance Minister Zhu Guangyao warned that military intervention "would have a negative impact on the global economy"
- The EU said there was "no military solution to the Syrian conflict".
France has strongly backed the US plan for military action. The French parliament debated the issue on Wednesday, although no vote was held.
On Wednesday, a US Senate panel approved the use of military force in Syria for 60 days with the possibility to extend it for 30 days.
The measure now goes to a full Senate vote next week and must also be approved by the US House of Representatives.
Mr Lahham, in his letter to Mr Boehner, invited a congressional delegation to Damascus and urged US congressmen to embrace civilised dialogue instead of the language of fire and blood.
He stressed the US and Syria have a common enemy - al-Qaeda and its affiliates who are playing an increasingly prominent role in the armed opposition in Syria.
The BBC's Jeremy Bowen in Damascus says the letter is similar to the one emailed to the speaker of the UK's House of Commons and copied to all the MPs just before they decided to vote against military action.
On Thursday the medical charity Medecins Sans Frontieres (MSF) said one of its surgeons, a Syrian working in Aleppo province, had been killed.
It gave no details of the circumstances but called for humanitarian workers to be protected.
Separately, Syrian rebels have launched an assault on the religiously mixed village of Maaloula, in western Syria, held by government forces.
A Christian nun in Maaloula told the Associated Press news agency that the rebels had seized a mountain-top hotel and were shelling the community below.