A worker turns a valve at a boosting compressor station ''...

UKRAINE - 2014/06/27: A worker turns a valve at a boosting compressor station ''East-Poltava'' near the village of Kovalivka, Poltava region some 357 kilometers of Kiev, Ukraine. Russia's state-controlled gas company, Gazprom, warned its European customers Friday that it could limit supplies to those European countries that intend to re-sell the natural gas on to Ukraine. (Photo by Iren Moroz/Pacific Press/LightRocket via Getty Images)