The prime ministers of Russia and Ukraine, Dmitry Medvedev and Mykola Azarov, had a phone conversation on Friday, during which Medvedev pointed out that Ukraine has to fulfill its obligations to pump gas into underground storage, the Russian government press service said.

"During the conversation, the Russian prime minister directed the attention of the Ukrainian [prime minister] to the necessity for Ukraine to meet its obligations to pump gas into underground storage in amounts sufficient for fulfilling international contracts," the press service said.

The two heads of government also discussed other points of trade and economic cooperation between their two countries.

The sides agreed to continue the discussion on this topic in St. Petersburg next week.

The day before, Gazprom Deputy CEO Vitaly Markelov warned of a "catastrophe," telling journalists that Ukraine would experience a 30% shortfall in the gas reserves in underground storage used to ensure transit of Russian gas to Europe.

Gazprom CEO Alexei Miller and Naftogaz Ukrainy chief Yevhen Bakulin met in Moscow on Thursday. "The parties addressed the bilateral cooperation issues. Special emphasis was placed on the repayment of the existing gas debt as well as the obligations to be fulfilled by Naftogaz Ukrainy for ensuring uninterrupted Russian gas transit to Europe in the forthcoming winter period," Gazprom said in a press release following the meeting.

"The Ukrainian colleagues, finding themselves in a difficult economic situation paying gas debts, have completely stopped buying Russian gas and begun taking gas out of underground storage in western Ukraine," Markelov said.

"The Ukrainian colleagues pumped into underground storage this year all of only 17.6 billion cubic meters, which is very little. And there are great risks to [them] getting through the fall-winter period. For uninterrupted gas transit 21.5 billion cubic meters had to be pumped. But now the Ukrainian colleagues have begun taking gas out of underground storage, when the winter period has not yet begun. At such outtake rates, Ukraine may have around 14 billion cubic meters left in underground storage when it starts getting cold. That is a catastrophe. In such conditions, the transit of gas from Russia in winter conditions is impossible for the reason that the gas in underground storage will not suffice to compensate for outtake by Ukrainian consumers," he said.