Moscow on Monday ruled out holding new negotiations with Ukraine over the price it pays for Russia's natural gas until Kiev covered its outstanding debts.
The announcement raises the possibility of Russia following through on its threat to cut off deliveries to Ukraine next month in a move that would endanger the supplies of at least 18 EU nations.
"We are saying that in order to discuss any sort of compromise, the debts must be paid first," news agencies quoted Russia's Deputy Energy Minister Anatoly Yanovsky as saying.
"Pay the debts and then we can agree on something."
Ukraine has refused to cover its obligations — put at $3.51 billion (2.55 billion euros) by the Russian energy ministry — in protest over Moscow's decision to nearly double the price it charges its neighbor for gas imports following the February overthrow of Kiev's Kremlin-backed regime.
Russia announced on Thursday that it will require Ukraine to pay upfront for all the natural gas it imports starting on June 1.
Ukrainian and Russian officials were due to meet behind closed doors with European Union counterparts in Brussels on Monday to discuss the possibility of arranging a ministerial meeting between the three sides on the gas dispute later this month.
The first round of negotiations conducted in Warsaw last week resulted in an agreement that Russia would not scale back its gas exports while the talks continued.
Nearly 15 percent of all gas consumed in Europe is delivered from Russia via Ukraine.
The danger for the EU is that Ukraine — its state coffers effectively empty and almost completely reliant on $17 billion promised by the International Monetary Fund -- will not cover its debt and instead start taking the gas Russia had earmarked for its European clients.
The nation of 46 million began dipping into supplies meant for Europe when it was cut off from Russian gas during previous price disputes in 2006 and 2009.