Russia's state-controlled energy company Gazprom said it filed a lawsuit against Ukraine in Stockholm arbitration court.
Gazprom said it wanted to recover the £2.7 billion ($4.5 billion) Ukraine owes it for its gas supplies.
Talks between Ukraine and Russia over their ongoing gas pricing dispute have ended with no deal reached, a spokesman for Russian producer Gazprom said.
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"People must be aware at all times that there are wild animals around them and they must make sure that their tents are properly closed."
The UK has been stripped of its top credit grade rating by Standard & Poor in the wake of its vote to leave the European Union.
Tonight: Showers will become confined to northern and western Scotland overnight leaving much of the country dry with clear spells.