Damascus. World leaders from G20 are meeting in St Petersburg, Russia, amid sharp differences over possible US military action against Syria, in response to what the US administration calls a deadly chemical weapons attack by the Syrian government.Thursday’s summit comes hours after a US Senate panel voted to give President Barack Obama authority to use military force against Syria - the first time lawmakers in that country have voted to allow military action since the October 2002 votes authorising the invasion of Iraq.The US and Russia, which is a key Syrian ally, remain at odds as Obama has tried to build his case for military action. The US president has vowed to continue to try to persuade his Russian counterpart, Vladimir Putin, of the need for punitive strikes against President Bashar al-Assad for using chemical weapons when the two meet in St Petersburg.As Putin opened the summit, he spoke exclusively about the global economic crisis, which forms the primary agenda of the summit, stressing the need for co-ordinated international policy making in order to combat continuing volatility in economic markets.He suggested that world leaders discuss the subject of Syria "during dinner” on Thursday night, so as not to take away from the summit’s primary economic agenda.Earlier, Putin had again questioned Western evidence justifying a military strike against Syria, accusing US Secretary of State John Kerry outright of lying when, in urging Congress to approve strikes, he played down the role of al-Qaeda in the rebel forces. ”Al-Qaeda units are the main military echelon, and they know this,” Putin said. "He is lying and knows he is lying. It’s sad.”Putin said US congressional approval without a UN Security Council resolution would be an act of aggression. He also told The Associated Press this week that he "does not exclude” supporting UN action - if it is proven that the Syrian government used poison gas on its own people.Obama has previously stated that he is prepared to bypass the UN Security Council on the issue, but has put the matter to a Congressional vote. Members of the full US Senate are due to debate the matter next week.The conflict in Syria, which began with a popular uprising in March 2011, has been stalemated, and it is not clear if US military strikes over the government’s alleged chemical weapons use would change that. Obama has said he seeks limited pinpoint action to deter future chemical attacks, not regime change.Meanwhile, China has warned other world powers of global economic risks following the potential US strikes on Syria.Speaking in St Petersburg ahead of the G20 summit on Thursday, Chinese Vice Finance Minister Zhu Guangyao said such "military action would definitely have a negative impact on the global economy, especially on the oil price”.He cited estimates that a $10 rise in oil prices could push down global growth by 0.25 percent.He urged a negotiated UN solution to the standoff over allegations that Syria’s government used chemical weapons against its own people, expressing hope that "the world economic balance will become more stable rather than more complex and more challenging.”Russia has also issued a warning that US strikes on Syria’s atomic facilities might result in a nuclear catastrophe and is urging the UN to present a risk analysis of such a scenario. The issue will be brought up at a meeting of the 35-nation IAEA board next week, the Interfax news agency reported.