Gazprom said a natural gas discount secured by the Ukrainian government in December may be canceled because of the $1.55 billion debt incurred by Naftogaz.
"What is owed is huge, not just for last year, but also debts for current deliveries," Gazprom spokesman Sergei Kupriyanov said in a Saturday interview with state news agency RIA Novosti.
Gazprom in 2009 cut gas supplies to Ukraine because of contractual disputes. A Ukrainian deal to settle the row put the price for natural gas at around $400 per thousand cubic meters, the standard purchase volume.
That was among the highest for Europe and the December deal for $268.5 per thousand cubic meters was meant to provide relief for a Ukrainian economy struggling with mounting debt issues.
European consumers get about a quarter of their gas needs met by Russia, though the majority of that gas is sent through a Soviet-era transit network in Ukraine.
Ousted Ukrainian President Viktor Yanukovych in November suspended efforts to sign free trade and association agreements with the European Union. That decision sparked a major uprising that led to him fleeing the country last month.
The new government says the economic situation in Ukraine is dire.