Gazprom in 2009 cut natural gas supplies to Ukraine following a contractual dispute. Prime Minister Yulia Tymoshenko helped broker the deal that ended the impasse, though she later was convicted on charges she abused her authority when negotiating the agreement.
The Ukrainian government last week said gas purchases from Russia would decline from around 975 billion cubic feet to about 700 billion cubic feet next year. Under a take-or-pay scheme, however, the former Soviet republic is required to pay Gazprom for its full volume.
Ukrainian Energy Minister Yuri Boiko was quoted by Russia's state-run news agency as saying his government had consulted legal experts on the matter.
"We will certainly argue with our Russian colleagues, maybe even in courts, but we know that we are absolutely right and we will act in accordance with our national interests," he was quoted as saying.
The bulk of Russian natural gas bound for Europe runs through Ukraine's gas transit system. Russian, along with European leaders, have pushed various natural gas pipeline projects that would circumvent Ukrainian territory.