Russia’s manipulation of gas prices under various pretexts has for a decade proved to be a particular headache for pro-Western Ukrainian governments.

Russia is now asking close to $500 for 1,000 cubic meters of gas, the standard unit for gas trade in Europe, which is a price about a third higher than what Russia’s gas company, Gazprom, charges clients elsewhere. Russia says the increase is justified because it seized control of the Crimean Peninsula, where its Black Sea naval fleet is stationed, ending the need to pay rent for the Sevastopol base. The base rent had been paid in the form of a $100 per 1,000 cubic meter discount on natural gas for Ukraine’s national energy company, Naftogaz.

Under an agreement signed in 2010 by Dmitri A. Medvedev, then Russia’s president and now the prime minister, the Russians lowered the price of gas in exchange for an extension of the base’s lease, effective for at least 25 years.

On Thursday, Mr. Medvedev said the discount was no longer warranted because Russia considered the entire Crimean Peninsula, including the base, to be its territory.