The government of Ukraine has rejected a bill of $US7 billion (5.1 billion euros) from Gazprom for unused gas that 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.
Ukraine says it used 27 billion cu m of Russian gas in 2012, but the contract stipulated payment for 52 billion cu m.
Gazprom sent the bill to Ukraine last month.
The BBC reports Russia has indicated to Ukraine that it can get gas at a lower price if it joins a customs union it set up with Kazakhstan and Belarus in 2010.
A foreign ministry official said last month that Ukraine ultimately had to choose between the union or EU membership.
Disputes over supplies to Ukraine previously saw Gazprom temporarily cutting off supplies.