Brussels/Strasbourg, France  - The European Union on Wednesday threatened to ditch Russia and Ukraine as gas suppliers and to encourage legal action against their state-owned energy companies unless they immediately resume shipments to Europe.

"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 on Monday in Brussels between Russia's Gazprom and Ukraine's Naftogaz on allowing international observers to monitor Russian gas flows destined to Western European clients via Ukraine.

The monitoring mission, which is composed of EU, Russian and Ukrainian experts, is designed to build trust between the two sides.

But despite having both vowed to resume supplies by Tuesday morning, no gas was reaching Europe by late Wednesday.

The latest hitch involved disagreements between Gazprom and Naftogaz over how to pump 100 million cubic metres of gas, representing about a third of what Gazprom's European clients normally receive, into Ukraine's pipelines.

Gazprom offered to send it to Ukraine's Sudzha station, but Naftogaz refused, saying it wanted to receive it at its Pisarevka and Valuyki compressor stations to the east.

"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."

Separately, Energy Commissioner Andris Piebalgs and Czech Industry and Trade Minister Martin Riman, whose country holds the rotating presidency of the EU, sent a letter to the energy ministers of Russia and Ukraine warning them that their countries' credibility as gas suppliers risked being "irrevocably damaged".

"If natural gas flows are not established immediately in full volumes, the credibility of Ukraine and Russia as reliable partners will be irrevocably damaged," the letter said.

Piebalgs's spokesman, Ferran Tarradellas, said Gazprom and Naftogaz were now facing "their last opportunity to limit the damages that they are doing to themselves."

Russia supplies about a quarter of the EU's gas needs, mostly through Ukraine.

And while Germany is by far Gazprom's biggest customer, the countries that have been most affected are those in the east, some of whom are totally reliant on Russian gas arriving via Ukraine.

The latest instalment in the three-week long dispute between Gazprom and Naftogaz prompted the prime ministers of Moldova, Bulgaria and Slovakia to travel to Moscow and Kiev on Wednesday in a desperate bid to find a diplomatic solution to their energy woes.

Russian President Dmitry Medvedev responded by inviting Russian gas importers to an emergency summit in Moscow.

In his address to the European Parliament, Barroso also pointed his finger at the EU member states' failure to make headway on the bloc's common energy policy.

He cited as evidence a commission proposal to use 5 billion euros (6.5 billion dollars) in unspent EU money to improve energy network connections that has not yet been agreed by national governments.

"Europe must act now to avoid future repetitions of these types of situations," Barroso said.

Czech Prime Minister Mirek Topolanek told parliament that the EU should learn from the crisis by further diversifying its supply routes.

But a 3,300-kilometre gas pipeline linking Azerbaijan to Austria and known as "Nabucco" is still at the planning stage and is not expected to be completed before 2018.

Topolanek also warned Ukraine that its behaviour in the crisis risked undermining its chances of becoming an EU member one day.

"This issue can influence the direction of the Ukraine, which has so far declared its pro-Western approach," Topolanek said. dpa