The European Commission has warned Russian and Ukrainian gas companies of legal action over a dispute which has left many Europeans without heat.
Commission president Jose Manuel Barroso said he would advise EU energy companies to sue unless Gazprom and Naftogaz move fast to restore supplies.
Bulgarian, Slovak and Moldovan prime ministers have held talks in Moscow and Kiev but the deadlock remains.
Supplies remain cut off because of a row between Russia and Ukraine.
'Matter of urgency'
Mr Barroso told the European Parliament the dispute between Ukraine and Russia was "most unacceptable and incredible".
He said if agreements, sponsored by the European Union, are not observed "as a matter of urgency", he would advise energy companies that have deals with Gazprom and Naftogaz, to file lawsuits against them.
" After 12 days, we will be obliged to resort to measures never seen in our history"
Despite optimism on Tuesday that the dispute was nearing an end, the rhetoric from Russia and Ukraine is increasingly antagonistic.
Moscow says gas is flowing again from Russia to Ukraine for export, but Kiev says it is technically unable to pass it on to its neighbours.
The Russian Prime Minister Vladimir Putin accused Ukraine of holding Europe hostage, countering Ukrainian accusations of Russian blackmail.
Eighteen other European countries have been affected by the gas supply row.
Hundreds of thousands of people are still without heating in eastern and south-eastern Europe after Moscow stopped supplies routed through Ukraine last week.
Bulgaria, one of the hardest hit countries, has had to close schools and other public buildings, due to the cuts.
The Bulgarian Prime Minister Sergei Stanishev told Mr Putin that millions of Europeans are "truly suffering".
Modolva's Prime Minister Zinaida Greceanii told the meeting in Moscow: "Our consumers do not understand why they should freeze when there is a contract for gas supplies."
Earlier, Slovakia's prime minister was in Kiev. He told his Ukrainian counterpart Yuliya Tymoshenko that his country had 11 days of gas reserves left.
Robert Fico said: "After 12 days, we will be obliged to resort to measures never seen in our history. "
Slovakia and the Baltic states are entirely dependent on gas from Russia.
Meanwhile, the Ukrainian President Viktor Yushchenko went to Poland, another country that relies entirely on Russia for its gas, to explain his case to his Polish counterpart.
Compromise difficult
An agreement, brokered late on Monday by the Czech prime minister, to allow international experts to monitor the flow of gas through Ukraine, was designed to overcome the deep feeling of mistrust between the two former Soviet neighbours.
Russia switched on the gas supply to Ukraine at the Sudzha pumping station on Tuesday morning.
But Ukraine said it could not physically pump the gas to Europe in the volumes Russia was providing, or along the pipeline route Moscow wanted it to take.
Russian energy giant Gazprom dismissed that claim. It says the taps are open on the Russian side.
The BBC's Gabriel Gatehouse in Kiev says it is now difficult to see how a compromise can be brokered.
Both Moscow and Kiev appear to be waiting for the other to admit defeat, our correspondent adds.
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