Japan has raised the severity of the nuclear crisis at its stricken Fukushima Daiichi plant to the highest level, putting it on a par with the Chernobyl disaster.
Officials said that radiation leaks were still far lower than those experienced during the 1986 nuclear disaster — the world's worst — but the situation remained unpredictable.
The warning came shortly after another powerful earthquake struck northern Japan, which is still struggling a month after a 9.0-magnitude quake and tsunami killed up to 28,000 people and triggered the nuclear crisis.
"Right at this moment, we are still trying to control this accident, and the nuclear reactors are not stable yet," said Hidehiko Nishiyama, a spokesman for Japan's Nuclear and Industrial Safety Agency.
"We are dealing with all our might and resources and try to minimize the impact of the radiation to the people around this nuclear plant."
Nishiyama said the decision to raise the accident to level 7 on an internationally-recognized scale was based on measurements of radiation around the stricken plant.
A level 7 incident indicates a major radiation release with the potential to cause environment and health problems over a wide area.
The disaster had previously been rated at level 5, on a par with the Three Mile Island incident in the United States in 1979.
Japan has expanded the evacuation zone around the Fukushima plant because of the high levels of accumulated radiation.
Chief Cabinet Secretary Yukio Edano said the change showed the scope of the disaster, but insisted the impact of radiation breaches at Fukushima were less severe than those at Chernobyl.
"The change in the level reminds us the accident is very big," he said. "I apologize to the residents of the area, the people of Japan and the international community."
He added: "What's different here from the Chernobyl accident is that we have not yet seen a direct impact on the health of the people as a result of the nuclear accident.
"The accident itself is big, but we will make, as our first priority, our utmost effort to avoid any health impact on the people."
Murray Jennex, a nuclear industry specialist at San Diego State University in California, told Reuters that the Fukushima disaster was still well below the scale of Chernobyl.
"It's nowhere near that level. Chernobyl was terrible -- it blew and they had no containment, and they were stuck," he said.
"[Japan's] containment has been holding, the only thing that hasn't is the fuel pool that caught fire."
Also Tuesday, Japan's economics minister warned the financial impact of the earthquake and tsunami disaster was likely to be far worse than feared.
"After a natural disaster, people tend to refrain from spending and you get a sense that factory output will shrink," Economics Minister Kaoru Yosano told reporters.
"In some areas, the impact could be very big."
On Monday another powerful earthquake struck about 100 miles northeast of Tokyo and about 30 miles southwest of the Fukushima plant.
The aftershock -— measured between 6.6 and 7.1-magnitude swayed buildings in Tokyo and triggered several landslides but did not cause a tsunami.