Hopes that Russia would get natural gas flowing again to shivering Europeans vanished like a winter's breath Tuesday when Moscow turned on the taps, then alleged hours later that Ukraine was blocking the shipments.

Ukraine in turn blamed Russia in the weeklong dispute, saying the Kremlin had demanded the Europe-bound gas go by a laborious route that would require Ukraine to cut off supplies to its own people. 

European Union monitors brought in to keep tabs on gas flows in both countries weren't saying who was at fault, but the EU was clearly angry at the crisis that has deprived millions of heat, light and even work.

"We are not entering a blame game here, but the commission reminds both parties of their responsibilities," EU spokesman Johannes Leitenberger said. "The European consumer cannot, shall not, be held hostage to what is clearly a bilateral situation."

Europe gets about 20 percent of its gas from Russia through pipelines that cross Ukraine. Russia stopped selling gas to Ukraine on Jan. 1 in a dispute over prices and debts, then stopped sending any gas into its vast pipeline system on Jan. 7, alleging Ukraine was siphoning off supplies destined for Europe.

The crisis raises high risks for both ex-Soviet countries. European countries that are already spooked by Russia's increasing military assertiveness – underlined by last summer's war with Georgia – could redouble their efforts to wean themselves from Russian gas. That would be a blow to Russia's already-struggling economy.

Ukraine risks angering the EU, which leaders in Kiev want deeply to join. Its refusal to pay market prices for Russian gas – one of the roots of the dispute – would make it look like a beggar in the eyes of the West. And if Ukraine is forced to cut off gas shipments to its eastern region in order to deliver gas to Europe, that could hit hard at its industrial heartland, which is also the power base for the opposition to Kiev's pro-Western leadership.

Russia's state-controlled gas monopoly Gazprom said it began pumping gas to Europe Tuesday. But EU officials said no gas was flowing and their monitors were not allowed full access to either nation's gas control rooms; Russia later brought monitors to Gazprom's central control room in Moscow.

Several European nations were growing desperate. Bulgaria has lost all of its gas supplies and has only two days worth of reserves. Slovakia, which has lost 97 percent of its gas supplies, said it was ready to restart an aging Soviet nuclear power plant despite EU objections.

Russian Prime Minister Vladimir Putin is to meet his Bulgarian and Slovakian counterparts in Moscow on Wednesday.