Ukrainian Prime Minister Arseniy Yatsenyuk has accused Russia of "economic aggression" and threatened to sue Russia over recent price hikes for natural-gas supplies.
"Russia was unable to seize Ukraine by means of military aggression. Now they are implementing plans to seize Ukraine through economic aggression," Yatsenyuk told a government meeting on April 5.
"Political pressure is unacceptable," he continued. "We do not accept the price of $500."
Yatsenyuk's comments come after Russia twice this last week raised the price of gas for Ukraine, taking the cost for Ukraine from $285.5 per 1,000 cubic meters at the start of this last week to $485.5 by the end of the week.
Energy and Coal Industry Minister Yuriy Prodan said attempts will continue to reach an acceptable gas price with Russia, but if that fails Ukraine will take the matter to arbitration court in Stockholm.
Gazprom: Ukraine Owes $11 Billion More
Earlier, Russian gas giant Gazprom said Ukraine must pay back $11.4 billion the country saved as part of a discount agreement that Moscow recently scrapped.
Gazprom Chief Executive Aleksei Miller said under the terms of the "Kharkiv Agreement" signed in 2009, Ukraine was able for the last four years to receive Russian gas at discounted rates.
Miller said that discount was a prepayment for the Russian Navy's use of Ukraine's Black Sea port of Sevastopol through 2017, which was part of the agreement.
Miller said that since that port had been annexed by Russia, along with the rest of Crimea, Ukraine should return the money it saved for gas shipments under the lower rate.
Miller also questioned the technical possibilities of reversing the flow of gas through pipeline systems and redirecting supplies from European Union countries to Ukraine.
Appearing on the Rossia-24 television channel on April 5, Miller said the idea of reversing the flow of gas in Europe would be difficult, since Europe's gas pipelines currently take gas from the east and send it west.
Miller said if it's possible to redirect Russian gas coming into Europe back toward Ukraine then there are legal questions that need to be answered.
While Russia raised gas prices for Ukraine $485.5 per 1,000 cubic meters, EU countries are still paying just under $400 for supplies of Russian gas.
With reporting by AFP, Interfax, Reuters, ITAR-TASS, and Rossia-24