Japan's nuclear safety agency has raised the Fukushima crisis level to five from four on the seven point international scale of atomic accidents.
The decision by the Nuclear and Industrial Safety Agency (NISA) puts Fukushima on the same level as the 1979 accident at Three Mile Island in Pennsylvania and makes it the worst ever in Japan.
A spokesman for the agency said it had alerted the IAEA on its decision, which was made due to the condition of reactors one, two and three at the plant stricken by last week's massive earthquake and tsunami that devastated northeastern Japan.
'The cooling function was lost and the reactor cores were damaged. Radioactive particles continue to be released in the environment,' he said.
Among the six reactors at the power plant, reactors one, two and three were operating at the time of the magnitude 9.0 quake and stopped automatically.
But the cores are believed to have partially melted because the disasters knocked out the plant's reactor cooling systems, sparking a series of explosions and fires.
Authorities have since struggled to keep fuel rods inside reactors and fuel storage containment pools under water.
If they are exposed to air, they could degrade further and emit large amounts of dangerous radioactive material.
France's Nuclear Safety Authority rates the Fukushima crisis at six on the scale. The Chernobyl disaster in 1986 is put at seven.
Japanese engineers have conceded that burying the nuclear plant in sand and concrete may be the only way to prevent a catastrophic radiation release.
International Atomic Energy Agency director general Yukiya Amano said efforts to stabilise the plant are 'a race against time'.
Officials said they still hoped to fix a power cable to at least two reactors to restart water pumps needed to cool overheating nuclear fuel rods.
Workers have also sprayed water on the No 3 reactor, one of the most critical of the plant's six.
It was the first time the facility operator had acknowledged that burying the sprawling complex - the method used to seal huge leakages from Chernobyl - was an option, a sign that piecemeal actions such as dumping water from military helicopters were having little success.
'It is not impossible to encase the reactors in concrete. But our priority right now is to try and cool them down first,' an official from the plant operator, Tokyo Electric Power Co, told a news conference.
Death toll from disasters reaches 6,539
The plight of hundreds of thousands left homeless by the earthquake and tsunami worsened following a cold snap that brought heavy snow to worst-affected areas.
Supplies of water, heating oil and fuel are low at evacuation centres, where many survivors wait bundled in blankets. Many elderly lack proper medical supplies. Food is often rationed.
The government said it is considering moving some of the hundreds of thousands of evacuees to parts of the country unscathed by the devastation.
The National Police Agency has said it has confirmed 6,539 deaths from the disasters, exceeding 6,434 who died after the Kobe earthquake in 1995. But 10,354 people are still missing.
Nearly 320,000 households in the north are still without electricity, Tohuku Electric Power Co said, and the government said at least 1.6m households are lacking running water.
The government has told everyone living within 20km of the plant to leave and advised people within 30km to stay indoors.
The US embassy in Tokyo has urged citizens living within 80km of the nuclear plant to move or remain indoors 'as a precaution'.
Britain's foreign office urged citizens 'to consider leaving the area'. Other nations have urged nationals in Japan to leave the country or head south.
Irish ambassador remains optimistic
The Tánaiste and Minister for Foreign Affairs, Eamonn Gilmore, said Irish citizens who wish to leave areas affected by potential radiation leaks are advised to do so.
However, Irish Ambassador to Japan John Neary has expressed optimism that the Japanese authorities will be able to stabilise the Fukushima plant, but added the embassy is closely monitoring the situation.
He said Tokyo remained in a 'very high state of anxiety' because people are concerned about possible impacts on the capital if things got dramatically worse at the plant.
Mr Neary restated that the advice to Irish citizens is to leave Tokyo 'for the time being'.
Asked whether it was safe to be in Tokyo, the Ambassador said that while radiation levels had increased 'slightly' on Tuesday, they had now returned to normal and were 'not a cause for concern'.
It is estimated there are around 2,000 Irish citizens in Japan and Mr Neary said many in the Tokyo region had either returned to Ireland or moved to the west or south of the country.
However, there are no precise statistics as how many Irish citizens are still here or where they are currently living.