A natural gas crisis has loomed over Europe, as a contract dispute between Russia and Ukraine shut off Russian gas supplies to six countries and reduced gas deliveries to several others.

At least two Bulgarian cities were totally without gas, and nations like Turkey were turning to Iran to bolster their supplies.

The European Union criticized the sudden cutoff to some of its member countries.

Johannes Laitenberger is the European Commission spokesman.

"Without prior warning and in clear contradiction with the reassurances given by the highest Russian and Ukrainian authorities to the European Union, gas supplies to some EU member states have been substantially cut. This situation is completely unacceptable."

In response, Russia and Ukraine have agreed to hold new talks on the contract dispute over Russian natural gas that travels to Europe through Ukraine's pipelines.

The two neighbors are locked in a dispute over pricing and overdue payments.

The energy dispute sharply escalated on Tuesday when six countries on the other end of the pipeline network reported a complete shutoff.

Bulgaria, Greece, Macedonia, Romania, Croatia and Turkey all reported a halt in gas shipments.