“The Ukrainian government has rejected a $7bn (£4.5bn; 5.1 euros) bill from Russia’s Gazprom for unused gas it was contracted to buy last year.
Energy Minister Eduard Stavitsky said there were “no grounds” to pay the money, which makes up nearly 4% of the country’s GDP ($183bn).
Ukraine says it used 27bn cu m of Russian gas in 2012 but the contract stipulated payment for 52bn cu m.
The country is under pressure to join a customs union with Russia.
A long-running dispute between the two neighbours over energy supplies appeared to have been resolved by an agreement in 2009, but Ukraine has since complained that an exorbitant price was fixed.