"We have faced an undisguised theft of Russian gas by Ukraine," Alexander Medvedev told journalists, adding that an independent international company has registered the theft.
"We are forced to use such terms as 'gas theft' as these facts have been registered by an independent international company. Despite Ukraine preventing specialists from accessing its gas measuring stations, measurements of incoming and outgoing gas [volumes] have led us to this conclusion," he said.
Medvedev said Ukraine has to observe the Russian gas transit contract valid for three more years. "There are no grounds to revise the transit contract," he said.
"There is only one way out - the lacking contracts on gas deliveries should be signed and Ukraine should ensure transit," he said.
"We are still ready to continue talks day and night. But even in this crisis situation, we don't see the Ukrainian side's desire to return to the negotiating table," Medvedev said.
Gazprom cut off gas supplies to Ukraine on Thursday after last-ditch talks with Kiev on a new deal for 2009 and debt repayments failed late on New Year's Eve.
Around 80% of Russia's gas exports to Europe pass through Ukraine.
Miller has accused Naftogaz of tapping Russian gas, which Ukraine denied, blaming Russia for creating the shortfall by deliberately cutting deliveries to Europe.
