Japan's nuclear safety agency on Friday raised the Fukushima crisis level to five from four on the international scale of gravity for atomic accidents, which goes to as high as seven.
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 swathes of 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 halted automatically.
But the cores are believed to have partially melted because the twin 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 is put at seven, the highest.
Level 3 indicates a "serious incident" according to the International Atomic Energy Agency (IAEA) scale, while level four means there has been an "accident with local consequences."
The 28 March, 1979 accident at Three Mile Island was a partial reactor meltdown that led to "very small" releases of radioactivity, according to the US Nuclear Regulatory Commission (NRC).
It caused no casualties, but was rated five on the scale, corresponding to "an accident with wider consequences."
Level 6 is a "serious accident".
The 26 April, 1986 explosion at the Soviet nuclear power plant in Chernobyl - rated a maximum 7 - was the world's worst nuclear disaster.
Unleashed by an unauthorised technical experiment, it spewed radioactive dust over swathes of Ukraine, Belarus, Russia and western Europe. Estimates of the death toll range from a UN 2005 figure of 4,000 to tens or even hundreds of thousands, proposed by non-governmental groups.