G20 leaders are meeting in Sankt Petersburg, Russia, amid sharp differences over the crisis in Syria.
US President Barack Obama has begun informal talks with other leaders as he pushes for military action over Syria’s alleged use of chemical weapons.
But Russian President Vladimir Putin has warned that military action without UN approval would be “an aggression”.
Syria is not officially on the G20 agenda in St Petersburg, but it is expected to dominate informal meetings.
The annual summit of the G20 group of the world’s leading economies is supposed to concentrate on the global economy.
Barack Obama, British PM David Cameron and Chinese President Xi Jinping are among the leaders who have now arrived at the G20.
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.
On the eve of the summit, a US Senate panel approved the use of military force in Syria, in response to an alleged chemical weapons attack.
The proposal, which now goes to a full Senate vote next week, allows the use of force in Syria for 60 days with the possibility to extend it for 30 days.
The measure must also be approved by the US House of Representatives.
The Damascus government is accused of using chemical weapons against civilians on several occasions during the 30-month conflict – most recently on a large scale in an attack on 21 August on the outskirts of the capital.
The government of Syrian President Bashar al-Assad has denied involvement and said the rebels were responsible.
The US has put the death toll from that incident at 1,429 – though other countries and groups have given lower figures – and says all the evidence implicates government forces.
Vladimir Putin dismissed as “ludicrous” claims the Syrian government used chemical weapons, but said Russia would be ready to act if there was clear proof of what weapons were used and by whom.
Barack Obama is trying to build support in the US for military action against the Syrian government.
After arriving in St Petersburg, he held talks with Japanese PM Shinzo Abe in the first of a series of meetings on the sidelines.
Barack 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.
Shinzo Abe has not stated publicly whether he supports military strikes.
A new study of images apparently from the chemical attack on August 21 concludes that the rockets carrying the gas held up to 50 times more nerve agent than previously estimated, the New York Times reported.
The study was carried out by an expert in warhead design, Richard Lloyd, and Theodore Postol, a physicist at the Massachusetts Institute of Technology.
The German intelligence service, the BND, told German MPs in a confidential briefing on Wednesday that Syrian forces might have misjudged the mix of gases in the attack, the German magazine Der Spiegel reported.
This might explain why the death toll was much higher than in previous suspected attacks, the head of the BND was quoted as saying.
France has strongly backed the US plan for military action. The French parliament debated the issue on Wednesday, although no vote was held.
The United Nations says more than 100,000 people have been killed since the uprising against Bashar al-Assad began in March 2011.
More than two million Syrians are now registered as refugees, the UN says, with an additional 4.25 million displaced within the country, making it the worst refugee crisis since the 1994 genocide in Rwanda.