Russian Prime Minister Dmitry Medvedev has instructed Gazprom CEO Alexei Miller to carefully monitor the situation surrounding Russian natural gas transiting Ukrainian territory.

At their meeting on Monday, Miller told Medvedev that at the end of December Ukraine paid off $1.65 billion as part of its debt for Russian gas shipments.

"These terms were secured as part of the trilateral agreements achieved between Ukraine, the European Commission and the Russian Federation (Gazprom). Consequently, Ukraine has been meeting all of its obligations concerning its debt re-payment," the Gazprom head said.

However, Ukraine has paid off only part of its gas debt, Miller said.

"Ukraine's unpaid gas bill stood at $5.3 billion. It means that they have paid off part of their debt to us," he added.

Ukraine also paid $150 million up front for Russian natural gas shipments in January, Miller said.

"Bearing in mind the fact that in December Ukraine used insignificant amounts of gas from its underground storage facilities, it did not collect the volumes of gas that were paid for in advance in December. Ukraine took only approximately 50% of the one billion cubic meters of gas in December. Consequently, taking the January $150-million pre-payment into consideration, Ukraine's amounts for January stand at around one billion cubic meters. Actually, it is not very much because it is getting colder and colder in Ukraine, and Ukraine will have to take twice as much gas from the pipeline in January as it did in December," Miller said.

The Gazprom also did not rule out possible situations in which gas could be collected illegally from the transit pipeline in Ukraine.

The risk of such situations remains primarily because Ukraine has not put sufficient amounts of gas in its underground storage facilities, he said.

"We know that the official figures were 16.5 billion cubic meters. It is not quite enough to last through the winter. Of the 16.5 [billion cubic meters of gas], 5 billion-6 billion is so-called "inactive" gas, which cannot be used in principle," Miller said.

"Naturally, we are interested in Ukraine acting as a reliable partner in this respect. But what you have said prompts us to think about how these events will develop further. I would like you to closely monitor how Ukraine collects gas, and, should such a need arise, promptly adopt decisions, if they are required," Prime Minister Medvedev said.