European Union nations may sue Russia and Ukraine unless the two countries quickly restore natural gas supplies to the bloc, the European Commission's president said on Wednesday.

Speaking at the European Parliament, Jose Manuel Barroso also said that unless gas started flowing soon he would advise the bloc's 27 member states to take measures to secure alternative long-term suppliers of energy.

"I would like to convey a very clear message to Moscow and Kiev. If the agreement sponsored by the EU is not honoured as a matter of urgency, the Commission will advise EU companies to take this matter to the courts," he said.

A dispute over gas between Moscow and Kiev has cut supplies of the fuel to European countries via Ukraine.

The gas has yet to start flowing again despite a recent agreement on transit brokered by the EU.

"The current situation is in short most unacceptable and incredible," Barroso said during a debate on the EU's programme for the first half of 2009.

"If the agreement is not honoured, it means that Russia and Ukraine can no longer be regarded as reliable."

Czech Prime Minister Mirek Topolanek, who holds the EU's rotating presidency for the first half of this year, told the debate that gas agreements must be made more transparent to avoid energy cuts in the future.