ST. PETERSBURG, Russia—Russian President Vladimir Putin vowed to continue supplying weapons and humanitarian aid to Syria in the event of a U.S. strike and rejected U.S. and European conclusions that Syrian government forces used chemical weapons in the suburbs of Damascus.

"Will we continue to help Syria? We will," Mr. Putin said after a 20-minute meeting with President Barack Obama at a summit here of the Group of 20. "We're helping them now. We are supplying arms, cooperating in the economic sphere."

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