Russian news agency ITAR-Tass said it was told by an official at Ukrainian state energy company Naftogaz it didn't make any payments for June gas by the Monday deadline because of price disagreements.
Ukraine lost out on a discounted price for natural gas in the wake of the November political upheaval that saw Kiev move closer to the European Union.
Russian energy company Gazprom in June said it would send natural gas to Ukraine only if paid in advance. Gazprom says Ukraine's gas debt is in excess of $5 billion.
Naftogaz has filed a case in an international court of arbitration challenging its natural gas debt to Gazprom.
Naftogaz was held under a take-or-pay contract with Gazprom, meaning it was obligated to a set volume of natural gas whether it used that gas or not.