Shareholders of a gas supply middleman that lost out after a January row between Ukraine and Russia said on Thursday Kiev had stolen 11 billion cubic metres of gas and demanded answers before pursuing a claim, according to Reuters.

Centragas, joint owner of RosUkrEnergo with Russian gas giant Gazprom (GAZP.MM), said Ukrainian state energy firm Naftogaz "expropriated" the gas in January while it was sitting in Ukrainian underground storage awaiting transit to Europe.

Russia and Ukraine rowed over gas prices and debts in January leading to several weeks of supply cuts that affected hundreds of thousands of Europeans. Europe gets a fifth of its gas supplies from Russia via Ukraine.

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The two countries signed a 10-year supply deal to end the row and scrapped RosUkrEnergo as an intermediary between Gazprom and Naftogaz. Ukrainian Prime Minister Yulia Tymoshenko and some experts had long accused RosUkrEnergo of corrupt practices.

"Centragas confirms that it has written to the Ukrainian Government to commence the process towards a claim under the Energy Charter Treaty relating to the expropriation ... by Naftagaz," a statement from Centragas reads. The statement did not specify in which court it wanted to pursue the matter and whether it was seeking compensation for the gas. It said the energy charter gives the two sides a three month consultation period.

Centragas is 90 percent owned by GDF, a holding company for Ukrainian tycoon Dmytro Firtash -- long a bitter rival of Tymoshenko. She, as other top politicians, is getting ready for a presidential election due on Jan. 17.

Neither Centragas nor Naftogaz were available to comment.

Tymoshenko said earlier this year when similar accusations of theft came up that Ukraine had bought the 11 billion cubic metres of gas for $1.7 billion from Gazprom, as half-owner of RosUkrEnergo.