Russia–Ukraine gas disputes

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.

Sources:  BBC  Sky News