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"If Russia's interests are ignored, we will also have to start reviewing the fundamentals of our relations," Putin told reporters in the Black Sea resort of Sochi.
In a declaration signed on Monday by Ukraine, the European Commission as well as three financial institutions, Kiev pledges to carry out reforms to ensure the sustainability, reliability, efficiency and transparency of its gas transit system, including the independence of the gas transit operators.
The declaration adopted in Brussels by Ukraine and the EU was poorly thought-out and uNPRofessional, Putin was quoted as saying by the RIA Novosti news agency.
Russian Energy Minister Sergei Shmatko said earlier that Russia had been excluded from talks in Brussels on the modernization of the Ukrainian gas pipelines.
Russia supplies a quarter of EU's gas needs, with 80 percent of the exports sent through Ukrainian pipelines. The energy supplier cut off gas supplies to Ukraine on Jan. 1 over a pricing dispute, and halted gas deliveries via Ukraine to Europe one week later, leaving millions of Europeans shivering in the depths of winter.