Japanese authorities have raised the severity rating of their nuclear crisis to the highest level, seven.
The decision reflects the total release of radiation at the damaged Fukushima Daiichi power plant, which is ongoing, rather than a sudden deterioration.
Level seven previously only applied to the 1986 Chernobyl disaster, where 10 times as much radiation was emitted.
There have been no fatalities resulting from the leaks at Fukushima, and risks to human health are thought to be low.
Meanwhile a 6.0-magnitude earthquake on Tuesday prompted the plant's operator to evacuate its staff.
The operator of the Fukushima plant, Tokyo Electric Power Co (Tepco), said it was checking the status of the plant after the quake, the second to hit in as many days, but said there had been no reports of problems with external power.
The aftershocks come a month after a huge quake and tsunami hit north-east Japan, leaving 13,219 people dead and 14,274 missing. More than 150,000 people have been made homeless.
Impact of leaks
An official from the Nuclear Safety Commission of Japan announced that the crisis level at the Fukushima Daiichi plant was being raised in a televised statement, adding that it was a preliminary assessment that was subject to confirmation by the International Atomic Energy Agency (IAEA).
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 (Nisa), the government's nuclear watchdog.
One official from Tepco said that radiation leaks had not stopped completely and could eventually exceed those at Chernobyl, Reuters news agency reported.
However, a nuclear safety agency spokesman told reporters the leaks were still small compared to those at the plant in Ukraine, then part of the Soviet Union.
"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 up to 630,000 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, reports said.
In comparison the Japanese government said the release from Chernobyl was 5.2 million terabecquerels.
The severity level of Japan's nuclear crisis had previously 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 millisievert in areas extending more than 60km (36 miles) to the north-west of the plant and about 40km 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.
Tepco said on Tuesday that a fire had broken out briefly at Reactor 4, before being extinguished.