Russia says Ukraine is stealing natural gas destined for Europe, as several countries have complained about a reduction of Russian gas deliveries from a pipeline passing through Ukraine. Kyiv has denied the charges, citing "technical difficulties" after Moscow cut-off supplies of natural gas to Ukraine amid a dispute over prices.
There is concern that many European families will be very cold this winter.
Russia's energy giant, Gazprom, says several of its key customers in Eastern Europe and the Balkans are receiving less Russian natural gas since it closed the taps to Ukraine on New Year's Day because of unpaid bills.
Gazprom's Chief Executive Officer Alexei Miller blames Russia's neighbor for the shortfall.
"Our European consumers in Hungary, Slovakia, Romania and Poland, and in the Balkans, are reporting unlicensed siphoning-off of Russian gas on Ukrainian territory," said Miller. "In this situation, Gazprom must provide additional volumes of gas through other gas transporting corridors."
Ukraine denies it is stealing from Russia's main pipeline to the West, saying there are technical reasons for Europe's natural gas reductions.
Ukraine's official on energy security, Bohdan Sokolovsky, has warned that Europe could face a bigger crisis within two weeks if Russia continues to halt natural gas deliveries to his country.
His comments were translated by Russia Today television.
"In around 10 days, there could be very serious technical problems," he said. "The transport of gas may be disrupted at some point and it will not be our fault."
Russia says natural gas shipments to Ukraine can resume only if it pays outstanding debts of more than two-billion dollars. Ukraine has pledged to pay only $1.5 billion.
In addition, Gazprom has raised the price of natural gas for Ukraine to $450 per 1,000 cubic meters - nearly twice the amount offered by Kyiv.
Gazprom and energy mediation company RosUkrEnergo accuse Ukraine of trying to blackmail the Russian gas producer into lowering prices.
RosUkrEnergo's Board of Directors member Hans Baumgartner says they are suing the Ukrainian government over the dispute.
"RosUkrEnergo itself filed two claims against Ukraine," he said. "To ensure gas transportation to Western Europe."
Ukraine has urged the European Union to mediate. Europe receives about a quarter of its gas supplies from Russia, but it has been reluctant to become involved in the Ukrainian-Russian dispute.
But the Czech Republic, which holds the rotating EU presidency, has called a special meeting of envoys from all 27 member states to discuss the crisis on Monday in Prague.