US Vice President Joe Biden on Saturday told Europe it was time to act on improving its energy security, warning of Russia's "track record" in using energy supplies as a weapon.
Speaking at an Atlantic Council meeting in Istanbul, Biden said Europe needed to step up efforts to "ensure diversification" away from Russian oil and gas imports.
He said the crisis in Ukraine had again showed up "Russia's track record in using energy supplies as a form of a political weapon against neighbours".
"It's a huge strategic problem," he told the conference.
Biden said that Russia "will and should remain" a major supplier of energy to Europe.
But he said: "Russia can be a player but must play by the rules of the game. It cannot play with the game."
He applauded steps by EU states like Lithuania and Slovakia in diversifying their supplies but said "we cannot rest on our laurels, we have to finish the job".
"Now is the time to act," he said.
Europe experienced one of its worst ever energy crises in 2009 when Russia switched off supplies to Europe through Ukraine in the middle of a freezing cold winter.