Russia has started feeding natural gas into a Baltic Sea pipeline that will deliver supplies to western Europe, while bypassing traditional transit routes in Ukraine.
Russian Prime Minister Vladimir Putin attended a special ceremony Tuesday to mark the launch of the pipeline in the city of Vyborg in northwestern Russia.
The pipeline, known as Nord Stream, connects Vyborg to the German port town of Greifswald but bypasses Ukraine.
Meanwhile, Ukraine says it is prepared to take Russia to court to prove it violated accords in a worsening gas dispute between the two countries.
Ukrainian officials are threatening to file an international arbitration suit against Russia's state-run energy company Gazprom, saying Ukraine is overpaying Russia by billions of dollars for imports from Gazprom.
There have been many price disputes between Moscow and Kyiv about natural gas deliveries to Ukraine, whose pipelines serve other European nations. In 2009, a disagreement between the two nations cut natural gas supplies to Western Europe during the cold winter months, leaving many without heat.
Nord Stream is a twin pipeline system. The first line can deliver about 27 billion cubic meters of gas per year.
Some information for this report was provided by AP, AFP and Reuters.