Russia has cut natural gas exports to Ukraine after the two countries failed to settle a price dispute.
Russia's state-owned gas company, Gazprom, wants to quadruple the price it charges Ukraine, saying Ukraine is a market economy and must pay market prices.
Kiev accuses the Kremlin of putting pressure on its economy with a sudden price hike. A Gazprom spokesman accused Ukraine Sunday of stealing gas headed to Europe through a trans-Ukrainian pipeline network.
Kiev has said a previous agreement with Moscow entitles it to 15 percent of natural gas moving across its territory.
Ukrainian President Viktor Yushchenko has assured Europe there would be no obstacles to Russian gas exports, but Poland is already reporting a shortage. The United States said it regrets the Russian decision.
The U.S. State Department said Moscow's action could have potential effects on gas supplies elsewhere in Europe, and raises "serious questions about the use of energy to exert political pressure." Washington is urging both sides to find a compromise.
Information for this report is provided by AP, AFP and Reuters.