The billing national joint-stock company Naftogaz Ukrainy by Russia's Gazprom is another evidence that Russia is trying to return to the full-scale fulfillment of the gas contract with Ukraine signed in 2009, according to former deputy prime minister of Ukraine Yuriy Boiko.

"The contract is unfavourable for us, as it includes very high prices of gas and registers the volume of gas it is required to buy," he told reporters in Kyiv on Thursday, commenting on the information on fines by Gazprom.

Boiko said that in 2013, the supplements to the contract which were accompanied by Kharkiv agreements were in effect and Russia did not imposed sanctions for not purchasing gas in the required volume by Ukraine.

"We've never paid for not purchasing gas in the required volume and we warned Russian partners at that time that we'd pump as much gas as the country needs," he said.

Boiko said he was confident that in the present situation the Justice Ministry should have a clear position and the sides are to hold professional negotiations.

As reported, Gazprom on April 23 presented Ukraine's Naftogaz with a bill of $11.388 billion, representing a penalty for the shortfall in gas purchases under pay-or-take terms in 2013, Gazprom Deputy CEO Alexander Medvedev told reporters.

The contract stated that Naftogaz should have bought 41.6 billion cubic meters of gas, but it only took 12.9 bcm.

The similar bill worth around $7 billion was served to Naftogaz Ukrainy at the end of 2012, although the bill was not paid and the Russian company did not file claims to court.