Japanese authorities have raised the measure of severity of their nuclear crisis to the highest level, officials say.
The decision was taken due to radiation measured at the damaged Fukushima Daiichi power plant, NHK reported.
The highest level for nuclear accidents (seven) had previously only applied to the Chernobyl disaster in 1986.
Meanwhile a 6.3-magnitude earthquake was reported off eastern Japan, the second tremor in as many days.
The aftershocks come a month after a huge quake and tsunami hit Japan, leaving nearly 28,000 dead or missing.
'Preliminary assessment'The raising of the severity level of the crisis at the Fukushima Daiichi plant was announced on national television by an official with the Nuclear Safety Commission of Japan.
"This is a preliminary assessment, and is subject to finalisation by the International Atomic Energy Agency," said an official at the Nuclear and Industrial Safety Agency (NISA), the government's nuclear watchdog, which made the announcement with the Nuclear Safety Commission, Reuters reported.
The level seven signifies a "major accident" with "wider consequences" than the previous level, officials say.
"We have upgraded the severity level to seven as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean," said Minoru Oogoda of Japan's Nuclear and Industrial Safety Agency.
However, a nuclear safety agency spokesman told reporters the leaks were still small compared to those of the Chernobyl accident.
"In terms of volume of radioactive materials released, our estimate shows it is about 10% of what was released by Chernobyl," he said.
The decision to raise the threat level was made after radiation of 10,000 terabequerels per hour had been estimated at the stricken plant for several hours.
That would classify the crisis at level seven on the International Nuclear and Radiological Event Scale (Ines).
It was not clear when that level had been reached. The level has subsequently dropped to less than one terabequerel an hour, the report said.
The severity level of Japan's nuclear crisis has so far been set at five, the same as that of the accident at Three Mile Island in the US in 1979.
Japan has also said it is extending the evacuation zone around the crippled nuclear plant because of radiation concerns.
The zone will be widened to encompass five communities beyond the existing 20-km (12-mile) radius, following new data about accumulated radiation levels, officials said.
Japan's nuclear commission said that according to preliminary results, the cumulative level of external radiation exceeded the yearly limit of 1 millisieverts in areas extending more than 60 kms (36 miles) to the north-west of the plant and about 40 km to the south-southwest.
On Monday, a 7.1-magnitude quake hit north-east Japan, leaving three people dead. It also triggered a brief tsunami warning, and forced workers to evacuate the Fukushima Daiichi plant.
Tuesday's quake rocked buildings in the capital, Tokyo.
There were no immediate reports of fresh damage, though Japan's Narita international airport temporarily closed its runways, and metro and train services were interrupted.
The cooling systems at the Fukushima Daiichi nuclear plant were damaged in last month's disaster and workers have been struggling to prevent several reactors from overheating.
Officials have warned it will be several months before the situation at the nuclear facility is brought fully under control.
Tokyo Electric Power Co (Tepco) said on Tuesday that a fire had broken out briefly at Reactor 4, before being extinguished.
The official death toll from the disaster is 13,130, while 13,718 remain unaccounted for. More than 150,000 people have been made homeless.
Analysis
Until now the incident at Fukushima was rated a level five, so upgrading to a level seven is a worrying jump.
But officials here point out that the change is not being made because of a sudden deterioration at the plant.
Rather a full assessment of the available data now suggests that Fukushima merits the higher rating according to internationally agreed standards.
There is no suggestion that the decision is related to a recent series of powerful aftershocks.
The move comes a day after the government announced it was extending the exclusion zone around the nuclear plant. Five communities where levels of radiation could pose a long term risk to health will be evacuated over the next month.
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