Gazprom reduced gas supplies by an additional 25% at 8:00 p.m. Moscow time [17:00 GMT] on Tuesday, bringing deliveries to half their normal level over Kiev's failure to pay its $600 million gas debt.
"We urge Ukraine to resume talks immediately," Gazprom spokesman Sergei Kupriyanov told a news conference. "If Ukraine fails to return to the negotiating table, I cannot rule out that a decision will be made to further reduce gas supplies."
The spokesman assured European consumers that Gazprom is continuing to supply gas to the EU in full, with gas shipments through Ukraine being monitored in Russia, Romania and Slovakia.
Since Monday's supply cut, independent observers have been barred from entering gas metering stations in Ukraine, according to Gazprom.
Kupriyanov noted that Ukraine's national oil and gas company Naftogaz has already warned that it could start siphoning off gas from supplies intended for European consumers. However, he said no reports of unauthorized gas consumption by Ukraine have yet emerged.
After the latest supply cut, the European Commission said that EU member states have so far not registered a shortfall in Russian gas supplies.
"To date, no Member State has reported any reduction in supplies," the executive branch of the EU said on its website.
Slovenian Economics Minister Andrej Vizjak, whose country holds the EU presidency, and EU Energy Commissioner Andris Piebalgs called in a joint statement on Tuesday for "a determined effort to resolve the current disagreement between Gazprom and Naftogaz."
Under a deal reached between President Vladimir Putin and his Ukrainian counterpart Viktor Yushchenko in Moscow on February 12, Gazprom and Naftogaz are to switch to direct cooperation without intermediaries, which Ukraine blames for allowing the debt to accumulate, and to set up two joint ventures. However no specific documents have yet been signed.