Japan has raised the severity level of its nuclear crisis to the maximum seven, putting the emergency at the Fukushima Daiichi power plant on a par with Chernobyl.
Officials from the nuclear and industrial safety agency (Nisa) confirmed that the crisis level had been raised from five to seven on the international nuclear and radiological event scale.
But they said the new rating reflects the initial impact of the nuclear crisis, adding that radiation levels have since dropped dramatically.
The scale, devised by the International Atomic Energy Agency, ranks nuclear and radiological accidents and incidents by severity from one to seven.
Level seven incidents involve a major release of radiation with widespread health and environmental effects, according to the IAEA.
In recent days Japanese officials had suggested there was no need to raise the severity level from five, which had been applied to the Three Mile Island accident in 1979.
A spokesman for Nisa said the decision to raise the level to the status of a major accident did not mean that the Japanese plant posed the same threat to public health or involved similarly big releases of radiation as the 1986 Chernobyl disaster.
"At Chernobyl, the reactor itself exploded while still active, which is completely different from the situation at Fukushima," Hidehiko Nishiyama said.
He added that the decision had been taken a month after the accident because experts needed time to analyse the data.
Japan's nuclear safety commission estimated that the Fukushima plant's reactors had released up to 10,000 terabecquerels of radioactive iodine-131 per hour into the air for several hours after they were damaged in the 11 March earthquake and tsunami.
The emission of radioactive substances from Fukushima Daiichi was about 10% of that detected at Chernobyl, Nishiyama said.
The nuclear safety commission said emissions have since dropped to below one terabecquerel per hour, adding that it was examining the total amount of radioactive materials released.
Some experts criticised the move as excessive. "I think raising it to the level of Chernobyl is excessive," said Murray Jennex, associate professor at San Diego State University. "It's nowhere near that level. Chernobyl was terrible – it blew and they had no containment, and they were stuck.
"The [Japanese] containment has been holding, the only thing that hasn't is the fuel pool that caught fire. I don't see those as the same event. If they want to do that, that's fine. I think they're being overly pessimistic."
Tuesday's decision came after the government said it would widen the evacuation zone near the plant to include five communities lying outside the current 20km no-go area.
About 70,000 people living within a 20km radius of the plant have already been evacuated, while 130,000 living between 20-30km from the plant have been told to leave voluntarily or stay indoors.
The latest evacuation, which could take at least a week to complete, was prompted by the lack of progress in fixing cooling systems at the damaged plant and concerns about the long-term effects on public health.
"These new evacuation plans are meant to ensure safety against risks of living [in affected communities] for half a year or one year," the government's chief spokesman, Yukio Edano, said.
Japan's north-east and eastern regions have been hit by two big aftershocks in the past 24 hours.
Shortly after 8am on Tuesday, an earthquake measuring magnitude 6.3 that struck off the coast of Chiba prefecture was followed by reports of a fire breaking out at the No 4 reactor at Fukushima Daiichi. The blaze was quickly extinguished, officials said.
It was one of more than 400 aftershocks above magnitude 5 to have hit the area since 11 March.
In one of the few signs of progress, the plant's operator, Tokyo Electric Power (Tepco), said it had stopped pumping low-level radioactive water from the reactor buildings into the sea.
The controversial measure, which drew criticism from neighbouring China and South Korea, was designed to free up storage space for highly contaminated water.
But engineers say they are no closer to restoring the plant's cooling system; until they do, they will be unable to cool overheating fuel rods and stabilise the facility's six reactors.
On Monday, Tepco's president, Masataka Shimizu, made his first visit to Fukushima prefecture since the crisis began.
"I would like to deeply apologise again for causing physical and psychological hardship," he said. The prefecture's governor, however, refused to meet him.