Leaders from the G20 group of nations have begun a summit in Russia, where they will discuss the Syria as well as the global economy. US President Barack Obama is pushing 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". Mr Putin said in his opening remarks that Syria, not formally on the agenda, should be discussed at a dinner later. Danger for aid workers Mr Obama, British Prime Minister David Cameron and Chinese President Xi Jinping are among the leaders at the G20 in St Petersburg. Mr Cameron, who lost a parliamentary vote on military intervention in Syria, told the BBC it was "the worst refugee crisis of this century". He called for aid agencies to receive more funding and for pressure to be put "on both sides in the conflict to improve access so aid workers can get to those who most need help". 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. Mr 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. Mr 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 Prime Minister Shinzo Abe in the first of a series of meetings on the sidelines. 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. Battle for middle ground A new study of images apparently from the chemical attack on 21 August 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. The BBC's Bridget Kendall, in St Petersburg, says both leaders have allies but the battle will be for the middle ground - those countries who share concern about chemical weapons but fear the consequences of military retaliation. 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 Mr 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.