The European Union on Wednesday threatened to ditch Russia's Gazprom and Ukraine's Naftogaz as gas suppliers and take court action unless shipments to Europe are immediately restored.
"Russia and Ukraine are showing they are incapable of delivering on their commitments to some European member states," European Commission President Jose Manuel Barroso told the European Parliament in Strasbourg.
"If the agreement sponsored by the EU is not honoured as a matter of urgency, the commission will advise European companies to take this matter to the courts and call on member states to engage in a concerted action to find out alternative ways of energy supply and transit," he said.
Barroso was referring to a deal signed Monday in Brussels allowing international observers to monitor Russian gas flows destined to Western European clients via Ukraine.
Despite the deal, Ukraine and Russia have failed to restore supplies, despite promising to do so by Tuesday morning.
"The current situation is both unacceptable and incredible," Barroso said.
"Unacceptable as European consumers in some member states are still without gas after a week without supplies. Incredible because we remain in the situation the day after an important agreement is signed at senior level with assurance from Russian and Ukrainian leaders that they will implement the agreement and let the gas flow."
Russia supplies about a quarter of the EU's gas needs, mostly through Ukrainian pipelines.
The commercial row between Gazprom and Naftogaz is believed to have strong political overtones due to Moscow's irritation at plans by Kiev to establish closer ties with Brussels.
"We will soon see whether there is a technical hitch, or whether there is no political intention to honour this agreement," Barroso said.
The commission head's comments were his strongest yet since the dispute started, at the end of 2008.