Vienna -- European countries vowed Wednesday to avoid a repeat of the crisis that saw crucial gas supplies cut off for 15 days due to a row between Russia and Ukraine, as they counted the cost of the dispute.

As Russian gas began flowing back into the millions of homes left without heat in the depths of winter, governments pledged to seek ways of preventing similar crises in future, after the cut-off highlighted the danger of over-dependence on a sole supply source.

"The current gas crisis has made it clear that we cannot allow ourselves to reject any source of energy," German Economy Minister Michael Glos said, hinting at a possible U-turn on his government's stated policy of phasing out nuclear power by 2020.

The European Union relies on Russia for almost a quarter of all the gas it consumes, notably for heating and for powering certain industrial activities.

Poland, which fared best during the crisis because it continued receiving more than 80 percent of its Russian gas via Belarus, unveiled plans to build "one or two" nuclear power plants by 2020.

Poland currently has no nuclear plants at all and generates 94 percent of its electricity from coal.

Prime Minister Donald Tusk said plans to build a new port for gas tankers would be brought forward, as would liquified natural gas (LNG) supply agreements with Qatar and Kuwait.

In Romania, President Traian Basescu called on the EU to unite to defend its energy interests.

"United action would enable the EU to better defend its interests and ensure an alternative" to Russian gas, he said.

He cited the EU-backed Nabucco gas pipeline project, a 3,400-kilometre (2,112-mile) pipeline between Turkey and Austria that will transport up to 31 billion cubic metres of gas each year from the Caspian Sea to western Europe.

Ministers from the six countries participating in the project are to meet in Budapest next week. And the project's leaders believe the crisis will provide sufficient momentum to clear key hurdles this year.

Hungarian Prime Minister Ferenc Gyurcsany -- currently on a trip to Oman and Qatar in search of alternative energy supplies -- hit out at Russia and Ukraine, accusing them of holding Europe hostage to what was effectively an internal pricing dispute between Moscow and Kiev.

Despite depending on Russian gas for more than 90 percent of its supplies, Hungary was able to fare reasonably well thanks to its substantial reserves of some 3.5 billion cubic metres, he said.

Serbia, however, had to call on emergency supplies from its neighbours.

Poor planning meant that its reservoir in Banatski Dvor, which would have helped buffer the country against the worst of the crisis, was almost empty when the Russian gas stopped coming.

Serbian industry estimated that 59 companies had been forced to scale down or completely shut down production, running up losses of 2.9 million euros per day for the companies themselves and as much as 5.0 million euros for the economy as a whole.

In Slovakia, Prime Minister Robert Fico estimated that industry had been losing 100 million euros a day last week.

And Economy Minister Lubomir Jahnatek said Bratislava would seek joint EU action for possible compensation from Russia and Ukraine.

Croatia is planning to build two more gas reservoirs in addition to its existing one, so as to have extra reserves in times of crisis, Prime Minister Ivo Sanader said.

Zagreb also wants to build a link to Hungary's pipeline network with EU money.

In Bulgaria, which is almost entirely dependent on Russian gas and was one of the countries worst hit by the crisis, the government put the direct losses from the crisis at around 169 million leva (86 million euros).

Prime Minister Prime Minister Sergey Stanishev said Bulgaria would join other EU countries in seeking compensation from Russia and Ukraine.

He said he believed the crisis would speed up the Nabucco pipeline project and said Sofia would throw its weight behind efforts to diversify energy supplies.

Simon Morgan/AFP/Expatica