Russia's Gazprom halts gas supplies to Ukraine until the Ukrainian government pays in advance for future supplies to replenish the current supplies of gas that it has used up; Ukraine says it has stopped buying from Gazprom because it can get cheaper gas from Europe. Roughly 15% of the gas used in Europe travels through Ukraine, and a previous cut-off of gas supplies in 2009 caused serious disruptions in shipments to EU countries at the height of winter.