"The transit risks in Ukraine remain this winter," Gazprom chief Alexei Miller was quoted as saying after meeting the new European Commissioner for Energy Union, Maros Sefcovic in Moscow.
Miller suggested that there could be a problem with Ukraine reducing transit supplies to Europe due to Kiev's problems with paying for enough gas for its own citizens to get through the winter.
Twice in the past decade Europe faced gas shortages during the peak winter heating season after Ukraine, which was locked in price disputes with Russia, allegedly took supplies destined for Europe from pipelines that cross its territory.
"Ukraine, due to financial difficulties, was not able to buy the necessary volume of Russian gas in November and December last year and significantly depleted its reserves of gas in underground storage," Miller said.
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Gazprom said for its part that it had met its obligations on supplies to Ukraine under a gas accord reached in November in Brussels but that Ukraine was not complying with its terms.
Ukraine's state gas company Naftogaz "is acquiring gas in lower volumes than in the Brussels accord," Gazprom complained.
Russian Prime Minister Dmitry Medvedev warned earlier Wednesday that Russia could demand rapid repayment of a USD 3.0 billion loan to Ukraine while the pro-Kremlin Viktor Yanukovych was still president in December 2013.
Russia agreed to give Ukraine a USD 15 billion loan but handed over only the first installment of USD 3.0 billion before Yanukovych was swept from power.
Medvedev, speaking at a business forum, complained that Ukraine in its budget had not made provision for paying the loan in its 2015 budget.
"We don't want Ukraine to default and for the already desperate position of the Ukraine economy to get worse," Medvedev said at a televised discussion on the economy.
"But debts need to be paid," he added.