The regrading to a "major accident" with "widespread health and environmental effects" puts Fukushima on a par with the world's worst ever peacetime nuclear event 25 years ago in the then Soviet Union.

The re-assessment came as Japan continued to be rocked by powerful aftershocks from the 9.0 magnitude quake that unleashed a devastating tsunami on March 11, with a 6.3 tremor striking Fukushima prefecture Tuesday afternoon.

Japan's Nuclear Safety Agency said radiation emissions from the Fukushima Daiichi plant, whose cooling system was knocked out by last month's disaster, were equal to 10 percent of the Chernobyl catastrophe.

Safety agency official Hidehiko Nishiyama said, however, that the two events were markedly different.

"In Chernobyl, there was acute exposure to a high level of radiation, and 29 people died from it. This is not the case in Fukushima," he said.

And while at Chernobyl the reactor itself had exploded, "in Fukushima... the reactors themselves have stayed intact, although we are seeing some leakage," the official said.

The earthquake and tsunami that crippled the Pacific coast plant in northeast Japan is confirmed to have killed 13,219 people, with more than 14,000 more still unaccounted for.

Nuclear experts have said a partial meltdown took place when the cooling systems failed, causing a series of explosions that leaked radioactive material into the atmosphere.

Tens of thousands of residents were evacuated from an exclusion zone covering a 20-kilometre (12-mile) radius from the plant and many more living close by have been advised to stay indoors.

On Monday the government said it would order people to leave certain areas outside the exclusion zone due to concerns over the effect of long-term exposure to radiation, but that a uniform extension of the zone was not appropriate.

Emergency crews at the plant have battled around the clock to bring the disaster under control and on Monday the government said the danger of a large leak of radioactive materials was becoming "significantly smaller".

Level seven incidents involve a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures", according to the UN's International Nuclear Events Scale.

Japan previously ranked the Fukushima crisis at five on the scale. Each increase in level indicates a roughly 10-fold increase in severity.

The meltdown at Chernobyl in the Ukraine spewed a large volume of toxic radiation, poisoning large areas of land and affecting thousands of lives.

The longer-term death toll from the accident ranges from a UN estimate in 2005 of 4,000 to tens or even hundreds of thousands, suggested by non-governmental groups.

Tuesday had earlier seen a 6.2 magnitude tremor hit 77 kilometres east of Tokyo, swaying buildings in the capital, temporarily shutting down subway services and halting bullet trains. US geologists originally put the magnitude at 6.4.

Japan has experienced more than 400 major aftershocks stronger than 5.0 in magnitude since March 11.

In a fresh setback, a fire broke out early Tuesday morning at a battery unit outside a building at Fukushima's No. 4 reactor, but was quickly extinguished and did not spread to other areas, plant operator Tokyo Electric Power said.

The operator said the blaze was not related to Tuesday's quake.

Frayed nerves were strained further Monday when coastal areas were put on alert for a possible tsunami after a 6.6 magnitude quake that killed three people in a landslide in Iwaki city, Fukushima.

Authorities withdrew the warning less than an hour later as the nation marked a month since the disaster, Japan's worst since World War II.

People around the country had fallen silent at 2:46 pm in remembrance of the victims.

Around 150,000 people are still in emergency shelters after losing their homes or being evacuated from around the leaking Fukushima plant.

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