There's no evidence to suggest Russian natural gas bound for Europe was siphoned off during transit through Ukraine, the Russian Energy Ministry said Monday.

"According to [the gas meters], there has not been any siphoning off of Russian gas in Ukraine," a ministry spokesman told state news agency RIA Novosti.

The Soviet-era gas transmission network in Ukraine carries more than half of the Russian natural gas bound for the European market. Russian energy company Gazprom last week said it would deliver gas to its Ukrainian counterparts only if paid in advance.

Gazprom switched to the pre-payment mechanism saying it was frustrated with the $4 billion debt for gas from Ukraine.

Gazprom CEO Alexei Miller said last week Ukraine would siphon off the gas for free unless it secured a substantial discount.

"Ukraine will not pay off its gas debt, will take our gas for free in the amount it needs," he said.

The Russian energy company said it was keeping close tabs on the gas metering system in Ukraine in an effort to spot any efforts to take gas from the transit system for Europe for free.