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.
