Slovakia is keeping the European Union on tenterhooks with its plan to reactivate an outdated nuclear reactor in a bid to avert an energy crisis after gas pipelines from Russia dried up.

The plan provoked angry reactions from the EU and environmentalists until Slovak Prime Minister Robert Fico said Monday he would wait another day, as Russia promised to open the gas taps on Tuesday morning.

Slovakia, a former communist country, closed the Soviet-type VVER-440/230 reactor at the Jaslovske Bohunice plant northwest of Bratislava for security reasons on December 31 to meet a pledge given to the EU before accession in May 2004.

"We met our obligation when we shut down Jaslovske Bohunice on December 31, 2008, but the ongoing gas crisis made us break it. When the crisis is over, we will start to comply with our obligations at once," said Prime Minister Robert Fico.

The country started preparatory work on the Jaslovske relaunch on Saturday as the Czech EU presidency was struggling to hammer out a deal on restoring deliveries from Russia via Ukraine.

"We will not avert the critical situation by resuming gas deliveries (...) we have more problems transmitting power than gas," said Economy Minister Lubomir Jahnatek, adding the country was facing the prospect of an energy "blackout."

But the European Commission in Brussels showed little understanding, saying on Monday the plan ran counter to EU law and "would be a clear violation" of treaties signed before Slovakia's accession.

"For us the reopening (of the nuclear reactor) is not an option," said commission spokesman Ferran Tarradellas.

Earlier on Monday, Fico insisted that his government would restart the reactor at the plant unless Russian gas supplies were restored to Europe on that day. Russia then promised to open the tap at 08H00 GMT on Tuesday.

Slovak officials fear the pressure in the local gas pipelines could fall to such a level that it would no longer be possible to supply gas to eastern Slovak households during the extremely severe winter.

EU Energy Commissioner Andris Piebalgs added to their woes by saying the gas "will take some time to reach our consumers, between 24 and 40 hours," while other EU officials have said it might take as much as three days.

Slovakia, which depends on Russia for 98 percent of natural gas imports, sent via Ukraine, declared a state of energy emergency last Tuesday to economise on its gas reserves, estimated to last for six days.

The Slovak energy system suffered another blow when a fire broke out at a coal-fired power station in Novaky on Monday, closing the facility for an estimated "21 days," according to Fico.

"This accident is yet another argument in favour of relaunching an old nuclear unit at the Jaslovske Bohunice plant" because different crisis scenarios such as this accident could occur, said the prime minister.

The perseverance upset Greenpeace environmentalists who warned Monday that the Slovak decision to restart the nuclear facility "would put the whole of Europe at risk."

"In Slovakia, natural gas is mostly used for heating. At most, it only generates six percent of electricity. Yet, nuclear power can only deliver electricity," said Jan Haverkamp, Greenpeace EU dirty energy policy campaigner.

"This is nothing more than a con by Slovak authorities," he added.