"Gazprom reduced supplies by 25% at 10 a.m. Moscow time (7 a.m. GMT)," Gazprom spokesman Sergei Kupriyanov said adding shipments to European consumers were continuing in full.
Ukraine's national oil and gas company Naftogaz confirmed the decline in supplies by 30 million cubic meters a day. The country receives 135 million cu m of gas a day from Russia.
In late February, Russia's gas monopoly threatened to cut shipments on March 3 if Ukraine, a major transit country for Russian gas to Europe, failed to pay off the debt and approve a new scheme for future supplies.
"Gazprom is a reliable supplier, but it cannot deliver gas without payment," Kupriyanov said. "The debt for gas supplied to Ukraine has not been paid off as of today. Ukraine is continuing the unauthorized consumption of gas. New contracts have not been signed."
Kupriyanov said around 1.9 billion cu m of gas worth some $600 million has been illegally consumed by the ex-Soviet state.
Ukraine said earlier it had paid over $1 billion to Gazprom to clear its debts for Russian supplies in 2007. Kiev owed Russia a total of $1.5 billion in 2007 and 2008, according to the Russian giant.
Kupriyanov said the monopoly was prepared to continue talks with Ukraine to resolve the dispute.
Under an agreement reached between Presidents Vladimir Putin and Viktor Yushchenko on February 12, Gazprom and Naftogaz are to switch to direct cooperation without intermediaries and to set up two joint ventures to this end.
A bitter gas pricing row between the former Soviet allies led to a brief cutoff of supplies to Ukraine at the start of 2006. Supplies to Europe, which buys a quarter of its gas from Gazprom, were also affected.
