European Commission President Jose Manuel Barroso issued a terse and direct message to Moscow's Gazprom and Kiev's Naftogaz to stop holding European consumers hostage to their contract dispute over natural gas prices.

"If the agreement is not honored, it means that Russia and Ukraine can no longer be considered reliable partners for the European Union in matters of energy supply," Barroso told the European Parliament.

He said if a solution to the energy crisis that has cutoff all gas supplies to Europe for a week was not implemented "as a matter of urgency," he would advise European energy companies to sue Gazprom and Naftogaz.

"The current situation is in short most unacceptable and incredible," Barroso added. "The fact is that Gazprom and Naftogaz are being unable to fulfill their obligations toward European consumers."

The tough language came a day after EU efforts to end the energy crisis faltered when a deal for having EU monitors check gas flows through Ukraine failed to restore supplies. Russia and Ukraine have blamed one another for the failure of that gas to reach Europe.

Russia supplies about one-quarter of the EU's natural gas, 80 percent of it shipped through Ukraine's vast pipeline network. Amid a pricing dispute with Ukraine, Russia cut off gas supplies to Europe on Jan. 7 just as the continent was gripped by freezing temperatures

Czech Prime Minister Mirek Topolanek, whose country currently holds the EU presidency, urged the 27-nation bloc to set up a 120-day "mandatory" strategic reserve of oil and natural gas and seek alternative energy supply routes. He called the situation a "crisis of trust" with Ukraine and Russia.

Topolanek proposed that the bloc make its "highest priority" plans to support the construction of the Nabucco natural gas pipeline, which would carry Caspian gas through Turkey to Europe, bypassing both Russia and Ukraine.

The Czech leader also called for diversification of energy sources, including developing more nuclear power.

"What we need to do now is to find a short term solution to end this shortage and we need in the long term to strive for new routes," Topolanek said. "The situation is very dire."

At least 15 European nations have seen their supplies stopped or reduced since Russia last week stopped sending gas through Ukrainian pipelines to countries further west.

Bulgaria has lost all supplies and has only a day or two of reserves. Slovakia has lost 97 percent of its supplies.

The two countries' premiers headed to Moscow for talks with top Russian officials on Wednesday to try to get a resumption of supplies.