The ongoing Russia-Ukraine stand-off may jeopardize natural gas supplies to European countries despite Moscow's efforts to abide by contracts, a senior official said Monday.

"Countries that have no alternative supply (routes) other than the Ukrainian gas transportation system are most at risk," Russian Deputy Energy Minister Anatoly Yanovsky told a press conference.

Yanovsky said Russian gas monopoly Gazprom had to demand prepayment from its Ukrainian counterpart, Naftogas.

"It is not a matter of Gazprom wanting or not wanting (to demand prepayment), it is an obligation under the contract. In order to supply gas to Ukraine, Gazprom has to pay custom duties to the Russian budget; that is why it simply must ask for prepayment," the RIA Novosti news agency quoted him as saying.

"We have not received any payments for the gas supplies already delivered (to Ukraine)," he added.

Also on Monday, a senior member of the Russian State Duma, the lower house of parliament, said energy supplies have become a political matter in recent years.

"Russia began supplies to Europe 40 years ago during the Cold War, but even then economic reasons and energy demands prevailed over political confrontations," said Pavel Zavalny, deputy chairman of the State Duma's energy committee.

Moscow has canceled two major contracts on gas discounts for Ukraine after the current authorities came to power in Kiev in February following President Viktor Yanukovych's ouster.

Gazprom earlier said Moscow's conflict with the authorities in Kiev could potentially hamper Russian natural gas deliveries to Europe through Ukraine. The company worries that Naftogas could fail on its transport obligations while withholding some gas allocated for Europe for Ukraine's own purposes.

Energy officials from the European Union are to meet Gazprom and Naftogaz representatives Monday in Brussels.