Russian giant Gazprom has slapped Ukraine's state gas firm Naftogaz with an unexpected $7 billion bill for gas it had not bought last year, a source in the Ukrainian firm said Saturday.
Media reports said the amount claimed by Russia's state natural gas supplier corresponded with a quantity of gas Ukraine should have imported in 2012 according to the contract between the two sides.
A source at Naftogaz, who did not wish to be named, confirmed that the company had received the bill but said Ukraine had already settled all payments for gas imported in 2012.
But the contract between Kiev and Moscow theoretically provides for a delivery of 52 billion cubic metres of gas and calls on Ukraine to pay for a large part of that, even if the country doesn't use all the gas.
Ukraine, which until last year was the main transit country for gas from Russia to Europe, has for months been trying to negotiate lower gas prices with Gazprom, so far in vain.
Russia has twice halted gas deliveries to Europe across Ukraine in recent years over its neighbour's outstanding debts and refusal to agree to higher gas prices.
In an effort to lessen its reliance on Russia, Ukraine is looking for alternative sources of energy. On Thursday, the ex-Soviet state signed a landmark $10-billion shale gas production sharing agreement with global oil giant Royal Dutch Shell.
Late last year, Naftogaz said Ukraine intended to reduce its Russian gas imports by buying gas from German supplier RWE.