MOSCOW/WASHINGTON (Reuters) - President Vladimir Putin warned on Thursday that Russian gas supplies to Europe could be disrupted if Moscow cuts the flow to Ukraine over unpaid bills, drawing a U.S. accusation that it is using energy “as a tool of coercion”. Russian President Vladimir Putin (R) chairs a meeting at the Novo-Ogaryovo state residence outside Moscow, April 10, 2014. REUTERS/Mikhail Kli