The crisis raises high risks for both Russia and Ukraine, ex-Soviet republics with deep historical ties but an increasingly troubled relationship.

European countries spooked by Russia's increasing assertiveness could redouble efforts to wean themselves from Russian gas. Czech Prime Minister Mirek Topolanek, whose country holds the EU presidency, proposed Wednesday that EU give its "highest priority" to a pipeline that would bypass Russia and deliver Caspian Sea gas via Turkey.

Russia, meanwhile, has used the gas dispute to push for new pipelines to Europe that would bypass Ukraine.

Underlying the energy dispute is a struggle between Russia and the West over the future of Ukraine, whose leaders have angered Russia by pressing for integration with NATO, supporting Georgia in its August war with Russia and threatening to evict a Russian naval base within a decade.

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Associated Press Writers Maria Danilova in Kiev, Ukraine, Constant Brand in Strasbourg, France, Steve Gutterman and Jim Heintz in Moscow and Monika Scislowska in Wisla, Poland, contributed to this report.