By Edmund Blair LONDON (Reuters) – Escalating tensions between the West and Russia over Ukraine have raised concerns about vital Russian oil and gas flows to Europe, prompting the United States to offer reassurances that it will help its European allies find alternative supplies. But that task may be mission impossible for Washington. Even without a build-up of Russian forces on Ukraine’s border and U.S. warnings of painful sanctions if tanks rumble across, the world’s oil and gas producers have struggled to meet global demand, pushing crude prices to their highest since 2014 and driving Europ…

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