The Russian state-run monopoly Gazprom, which has halted supplies to Ukraine over a pricing dispute, says all Russian gas now going through pipelines in Ukraine is intended for European customers.
Ukrainian authorities have denied Russian charges that Ukraine has started to illegally siphon away gas intended for European users.
Germany, Italy, France, and Austria have made a joint appeal to Ukraine and Russia to ensure that gas flows to Western Europe remain steady. Gazprom supplies some 25 percent of Western Europe's gas, most of it via Ukraine.
The U.S. State Department said it regretted Russia's move to halt supplies to Ukraine, saying the move creates energy insecurity and raises serious questions about the use of energy to exert political pressure.
Ukrainian President Viktor Yushchenko has called for Russia to return to negotiations to settle the crisis. Yushchenko has said Ukraine is prepared to pay more for Russian gas, but cannot immediately jump from paying $50 per 1,000 cubic meters to $230, as demanded by Gazprom.