Gazprom spokesman Sergei Kupriyanov told news agencies that "we have been forced to start the operation to lower the pressure in Ukraine's gas pipeline system."
He added that supplies of Russian gas exported via Ukraine to Western Europe are not affected, saying that "export gas will be delivered in its totality." More than 80 percent of Russia's natural-gas sales to Europe travel through Ukrainian pipelines.
However, Poland is reporting a fall in pressure in its gas pipelines entering the country from Ukraine.
Kupriyanov accused Ukrainian officials of seeking to compensate for the shutoff by siphoning off gas bound for Europe. "The Ukrainian authorities were inclined to have a conflict from the start, and from January 1st to start the unsanctioned extraction of gas – or, to be more precise, to start stealing it, stealing it from European consumers," Kupriyanov said.
Russia's Foreign Ministry has cautioned Kyiv against siphoning off natural gas earmarked for other European countries.
No official reaction from Ukraine was immediately available. However, the spokesman for Ukraine's state-owned oil and gas company Naftohaz Ukrayiny, Eduard Zanyuk, confirmed that pressure in the pipelines is "getting lower."
Naftohaz had warned that a shutoff would threaten gas deliveries to Europe.
Gazprom said it cut gas supplies after Ukrainian authorities had rejected a final offer from Moscow that would have ensured the supply of gas through 2006.
Gazprom had said it would turn off the taps to Ukraine at 10:00 Moscow time unless Kyiv agreed to higher prices.
Russian President Vladimir Putin had offered to maintain the current price of $50 per 1,000 cubic meters until April on condition that Ukraine would then accept higher prices, possibly up to $230 per 1,000 cubic meters.
Ukraine has previously said it was unwilling to pay the higher price demanded by Gazprom.