Washington and Russia remained publicly at odds over Syria on Wednedsay with President Vladimir Putin accusing U.S. Secretary of State John Kerry of lying by playing down the role of al-Qaida with rebel forces.
"He is lying and knows he is lying. It's sad."
Kerry played down concerns that any U.S. military strike over chemical weapons might provoke a clash with Russia.
"Foreign Minister (Sergei) Lavrov has made it clear ... Russia does not intend to fight a war over Syria," Kerry told a hearing of the House Foreign Affairs Committee.
Obama said he would continue to try to 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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Putin again questioned Western evidence on a chemical weapons attack by Syrian President Bashar Assad, but said in an interview with The Associated Press that he could not absolutely "rule out" Russia supporting a U.N. Security Council resolution to punish Assad - if it could be proved he had used poison gas.
Briefing members of Congress in Washington, Kerry said those comments were "hopeful" and "there may be a road forward where Russia would consider not blocking action."
A senior Western official said that while Moscow was unlikely to say so in public, there were signs Russian officials believe Assad was 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.
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