The Japanese government says it was overwhelmed by the scale of to the huge earthquake and tsunami, slowing the response to the nuclear crisis.
The twin disasters have left at least 10,000 people dead.
The admission came as Japan welcomed US help in stabilising its overheated, radiation-leaking nuclear complex at Fukushima on the north-east coast.
It also reclassified the rating of the nuclear accident from Level 4 to Level 5 on a seven-level international scale, putting it on a par with the 1979 Three Mile Island accident.
Nuclear experts have been saying for days that Japan was underplaying the severity of the nuclear crisis, which the prime minister today called “very grave”.
The International Nuclear Event Scale defines a Level 4 incident as having local consequences and a Level 5 as having wider consequences.
Hidehiko Nishiyama of Japan’s nuclear safety agency said the rating was raised when officials realised that at least 3 % of the fuel in three of the reactors at the Fukushima Dai-ichi plant had been severely damaged, suggesting those reactor cores have partially melted down and thrown radioactivity into the environment.
“The unprecedented scale of the earthquake and tsunami that struck Japan, frankly speaking, were among many things that happened that had not been anticipated under our disaster management contingency plans,” said Chief Cabinet Secretary Yukio Edano, admitting that information had not been shared quickly enough.
“In hindsight, we could have moved a little quicker in assessing the situation and co-ordinating all that information and provided it faster,” he said.
Later, Prime Minister Naoto Kan urged the nation to unite.
“We will rebuild Japan from scratch. We must all share this resolve,” he said in a nationally televised address, calling the crises a “great test for the Japanese people.”
At the stricken complex, military fire trucks sprayed the reactor units for a second day, with tons of water arcing over the facility in desperate attempts to prevent the fuel from overheating and spewing dangerous levels of radiation.
“The whole world, not just Japan, is depending on them,” Tokyo office worker Norie Igarashi, 44, said of the emergency teams working amid heightened radiation levels at the complex.
Last week’s 9.0 quake and tsunami set off the nuclear problems by knocking out power to cooling systems at the Fukushima plant on the north-east coast. Since then, four of its six reactor units have seen fires, explosions or partial meltdowns.
The unfolding crises have led to power shortages in Japan, forced factories to close, sent shockwaves through global manufacturing and triggered a plunge in Japanese stock prices.
“We see it as an extremely serious accident,” Yukiya Amano, the head of the UN’s International Atomic Energy Agency, told reporters in Tokyo.
“This is not something that just Japan should deal with, and people of the entire world should co-operate with Japan and the people in the disaster areas.”
“I think they are racing against the clock,” he said of the efforts to cool the complex.
One week after the twin disasters – which have officially left more than 6,900 dead and more than 10,700 missing – emergency crews are facing two challenges in the nuclear crisis: cooling the reactors where energy is generated, and cooling the adjacent pools where used nuclear fuel rods are stored in water.
Both need water to stop their uranium from heating up and emitting radiation, but with radiation levels inside the complex already limiting where workers can go and how long they can remain, it has been difficult to get enough water inside.
Water in at least one fuel pool – in the complex’s Unit 3 – is believed to be dangerously low. Without enough water, the rods may heat further and spew out radiation.
“Dealing with Unit 3 is our utmost priority,” Edano told reporters.