U.S. President Barack Obama said he would continue to try and persuade Putin of the need for punitive strikes on Assad for using chemical weapons when the two meet in St. Petersburg.
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But Putin again questioned Western evidence. And he accused U.S. Secretary of State John Kerry outright of "lying" when, in urging Congress to approve strikes on Syria, Kerry 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."
Having surprised friends and foes alike by seeking approval from Congress before attacking, Obama has been building support ahead of votes in Washington expected next week. In Stockholm en route for Russia, he appealed to lawmakers' consciences:
"America and Congress's credibility is on the line," he said. "The question is how credible is Congress when it passes a treaty saying we have to forbid the use of chemical weapons."
Earlier, Putin had said in a pre-summit interview with the Associated Press that he could not absolutely "rule out" Russia supporting a UN Security Council resolution to punish Assad - if it could be proved he had used poison gas.
A senior Western official said that, while Moscow was unlikely to say so in public, there were signs Russian officials believe Assad was indeed responsible for the deaths on Aug. 21 and that it had strained Russian support for him - providing an opening for a new, concerted drive to end the conflict.
However, Putin's characteristically blunt tone towards the U.S. position appeared to limit prospects for a breakthrough in a stalemate that has prevented international action to rein in a conflict that has killed more than 100,000 Syrians and left millions homeless but which neither side has been able to win.
"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 a news conference in Sweden.
Obama rejected suggestions he set a red line for action against Syria in the event Assad used chemical weapons. He said the red line was set by the international community and by Congress which ratified a treaty condemning the use of chemical weapons. He acknowledged that a military response will not resolve the civil war in Syria, but is needed to send a clear message.
Obama arrived in Sweden on Wednesday en route to the Group of 20 (G20) economic summit in Russia where the Syrian civil war is expected to dominate the discussions.
Obama is the first incumbent U.S. president to make a bilateral visit to Sweden. The trip was announced in August, shortly after he cancelled a meeting with his Russian counterpart following Moscow's asylum offer to fugitive U.S. whistleblower Edward Snowden.
A massive security detail was in place in the Swedish capital, Stockholm, for the roughly 24-hour visit. An estimated 2,000 police officers, some called in from other parts of the country, were part of the detail with U.S. security agents.
"No to Big Brother Obama" was the overarching theme of a protest organized by the September 4 Network critical of US surveillance operations disclosed by Snowden and others.
While Obama was en route to Stockholm, Russian President Vladimir Putin hinted for the first time that Moscow might back a UN resolution for military intervention in Syria.
That conflict was also likely to feature during Obama's meeting with Swedish Prime Minister Fredrik Reinfeldt, who has invited Nordic leaders to a working dinner.
Of the attending leaders from Denmark, Finland, Iceland and Norway, only Danish Prime Minister Helle Thorning-Schmidt has said her government could support possible military action against Syria even without UN Security Council approval.
Reinfeldt told Swedish television, shortly before Air Force One touched down at Arlanda Airport at about 0800 GMT, that he planned to discuss free trade, citing Sweden's dependence on foreign trade and exports.
Sweden is a member of the European Union that recently opened talks on a free trade deal with the United States.
Obama will next travel to the Russian city of St Petersburg on Thursday for the G20 summit hosted by Putin.