Japan has upgraded its radiation severity level on a par with the world's worst nuclear accident Chernobyl.
Radiation leaked from the earthquake-crippled Fukushima Daiichi plant in the early days of the crisis has now been re-measured, although Japanese officials said the upgrade in its severity rating from level 5 to 7 did not mean the situation had suddenly become more critical.
"Our preparations for how to measure the radiation leakage when such a tsunami and earthquake occurred were insufficient and, as a result, we were late in disseminating information internationally," said a senior official in Japanese Prime Minister Naoto Kan's office.
The severity rating at level 7 is the same as the Chernobyl disaster in Ukraine in 1986 and was based on cumulative quantities of radiation released, according to Hidehiko Nishiyama, a deputy director-general of the Nuclear and Industrial Safety Agency (NISA).
"Before this we had considered this a very serious incident so there will be no big change in the way we deal with it just because it has been designated level 7," an agency official said.
After another major aftershock a fire broke out at the Fukushima plant, but engineers managed to put out the blaze.
However, the plant operator Tokyo Electric Power Company (TEPCO) seems to be no closer to restoring cooling systems at the reactors, critical to lowering the temperature of overheated nuclear fuel rods.
Prime Minister Kan is expected to instruct TEPCO to set target dates for when it would halt the radiation leakage as well as restore the cooling systems.
No radiation-linked deaths have been reported since the earthquake struck, and only 21 plant workers have been affected by minor radiation sickness, according to chief cabinet secretary Yukio Edano.
A level 7 incident means a major release of radiation with a widespread health and environmental impact, while a 5 level is a limited release of radioactive material, with several deaths, according to the International Atomic Energy Agency (IAEA).
Several experts said the new rating exaggerated the severity of the crisis, and that the Chernobyl disaster was far worse.
"It's nowhere near that level. Chernobyl was terrible - it blew and they had no containment, and they were stuck," said nuclear industry specialist Murray Jennex, an associate professor at San Diego State University in California.
"Japan's containment has been holding and the only thing that hasn't is the fuel pool that caught fire."
The blast at Chernobyl blew the roof off a reactor and sent large amounts of radiation wafting across Europe.
The accident contaminated vast areas, particularly in Ukraine and neighbouring Belarus, and led to the evacuation of well over 100,000 people as well as affecting livestock as far away as Scandinavia and Britain.
Increasing the severity level however could spark diplomatic tension with Japan's neighbours over radioactive fallout.
"Raising the level to a 7 has serious diplomatic implications," said Kenji Sumita, a nuclear expert at Osaka University.
"It is telling people that the accident has the potential to cause trouble to our neighbours."
The earthquake and tsunami killed up to 28,000 people and the estimated financial cost stands at $300 billion, making it the world's most expensive disaster.
The damage was likely to be worse than first thought as power shortages would cut factory output and disrupt supply chains, warned Japan's economics minister.
The economy is in a "severe state" according to The Bank of Japan governor, while central bankers are uncertain if efforts to rebuild the north-east will boost growth.
The amount of radiation released into the atmosphere from the plant, 240 km north of Tokyo, was around 10 per cent that of Chernobyl, NISA said.
"Radiation released into the atmosphere peaked from March 15 to 16," said NISA's Nishiyama. "Radiation is still being released, but the amount now has fallen considerably."
This level of radiation is harmful, according to Lam Ching-Wan, a chemical pathologist at the University of Hong Kong and member of the American Board of Toxicology.
"It means there is damage to soil, ecosystem, water, food and people. People receive this radiation. You can't escape it by just shutting the window," Lam said.