"If the Ukrainian side does not settle the problem on Monday, Gazprom will have to stop supplies of Russian gas to Ukraine," Gazprom spokesman Sergei Kupriyanov said.
He said Russia had increased gas supplies to Turkey, Greece and Ukraine since the beginning of the year after those countries faced lower deliveries from other suppliers.
"All our partners follow payments schedules, and it is only with Ukraine that a paradoxical situation has emerged where it gets the gas it needs, but does not pay for it," he said.
Gazprom has told the European Commission that supplies to the EU will not be affected in its dispute with Ukraine, EU Energy Commissioner Andris Piebalgs said Thursday.
"Gazprom has today contacted the European Commission to inform us about a commercial problem with Ukraine that may lead to the interruption of Russian gas supplies to Ukraine," Piebalgs said in a statement. "Gazprom has given assurances that gas supplies to the [EU] will not be interrupted."
Gazprom supplies Europe with one-quarter of its gas via Ukraine and Belarus. Russia's ambassador to Kiev, Viktor Chernomyrdin, declined to comment Thursday, saying he had not heard anything about the threat, Interfax reported.
"Who said that? I don't know anything. ... You say a statement was made? I would have known," he said, Interfax reported.