STOCKHOLM - President Barack Obama issued a blunt challenge to skeptical US lawmakers on Wednesday to approve his plan for a military strike on Syria, saying otherwise they would put America's international prestige and their own credibility at risk.Using a visit to Sweden to build his case for limited military action against Syrian President Bashar Assad, Obama insisted that the international community could not remain silent in the face of the "barbarism" of the August 21 chemical weapons attack he blamed on Syrian government forces."My credibility is not on the line. The international community's credibility is on the line," Obama told a news conference in Stockholm. "And America and Congress' credibility is on the line, because we give lip service to the notion that these international norms are important." Just a day before he travels to St. Petersburg to attend a G20 summit hosted by Vladimir Putin, Obama said he held out hope that the Russian president would back away from his support for Assad. But he stopped short of saying he had any high expectations for a change of heart.Obama's comments came after Putin offered a glimpse of potential international compromise over Syria on Wednesday by declining to entirely rule out Russian backing for military action as he prepared to host a summit of world leaders. At the same time, Putin said any strike on Syria would be illegal without UN support.Obama has taken a big political gamble by delaying military action in Syria and instead trying to convince a divided US Congress to grant authorization for a strike on government targets in Syria.Aides say that even as Obama travels he will stay on top of the congressional debate raging back in Washington, where his national security team has waged an intensive campaign to ease the concerns of reluctant lawmakers and a war-weary American public.While declaring that he believes Congress will give him approval, Obama ratcheted up the pressure for swift legislative action, saying inaction could embolden Assad to carry out further attacks. "The question is how credible is Congress when it passes a treaty saying we have to forbid the use of chemical weapons," Obama said.WILL OBAMA ACT ALONE?Obama declined to say whether he would proceed with a military strike even if Congress rejected the plan. But he said he was not required by law to put the matter before Congress and made clear he reserves the right to act to protect US national security.Obama will fly to St. Petersburg on Thursday to take part in an annual two-day summit of the Group of 20 leading economies, a gathering sure to be dominated by tensions over Syria.The meetings will bring him face-to-face with Putin, a key Syrian ally, the main arms supplier to Damascus and a staunch critic of the US push for military action."Do I hold out hope that Mr. Putin may change his position on some of these issues? I'm always hopeful, and I will continue to engage him," Obama told reporters. Putin said Wednesday that Russia is not yet prepared to accept US and European assertions that Assad's forces were behind the chemical weapons attack that Washington says killed more than 1,400 people."We have no data that those chemical substances - it is not yet clear whether it was chemical weapons or simply some harmful chemical substances - were used precisely by the official government army." Syria tops the list of disputes that have sent US-Russian ties to one of their lowest points since the end of the Cold War.Obama's three-day foreign trip offers a chance to lobby world leaders for their support and possibly shore up a shaky international coalition against Syria.Britain, a generally reliable US ally, pulled out after a parliamentary revolt last week, but France, western Europe's other main military power, is still coordinating possible action with the Pentagon.Any attack on Syria is likely on hold until at least next week, the earliest timeframe for a vote by lawmakers, who formally reconvene on September 9 after their summer break.Obama faces a tough fight in Congress for endorsement of military action over what Washington says was the killing of 1,400 people in a chemical weapons attack carried out by Assad's forces.Many lawmakers staunchly oppose a strike, fearing it would entangle the United States in the seemingly intractable Syrian civil war. Others favor rewriting the use-of-force resolution the White House sent to Capitol Hill over the weekend so that it sets clear limits on any military action.Still others, mostly more hawkish Republicans, want Obama to make sure any strike is punishing enough to weaken the Syrian military, and are calling for increased help for beleaguered anti-Assad rebels.