What is the argument of Russia as far as the natural gas selling is concerned?

Russia’s state-controlled gas company says it will increase the price of natural gas it sells to Ukraine. Both countries are arguing over natural gas. Russia says Ukraine is stealing it and Ukraine says Russia won’t negotiate honestly. Any price increase, and Ukraine says, should come with a hike in the money it gets for letting Russia use the pipelines to begin with. Relations are strained between both countries over last summer’s Russian invasion of Georgia.