"What are they thinking about? This is insanity in this situation to leave the country without deals for 2009," Viktor Zubkov, who chairs natural gas monopoly Gazprom's board of directors, told the Vesti TV channel.
He said Russia is ready to take appropriate measures if Ukraine fails to pay its $2 billion-plus debt for Russian natural gas supplies, set as a condition for a new gas deal.
Zubkov said he had sent letters to European leaders warning them that Russia would deny the transit country further gas supplies if the debt was not settled by the end of this year.
He reminded EU leaders that Ukraine had been siphoning off Europe-bound gas in early 2006 after Russia was forced to cut off supplies to the country amid a pricing row.
According to Zubkov, Europe had agreed that "the ball is in Kiev's court."
A delegation from Ukrainian national energy company Naftogaz is currently holding talks in Moscow.