Japan's Nuclear and Industrial Safety Agency Tuesday raised the level of severity at the Fukushima Daiichi nuclear power plant from 5 to 7 – the highest level on the international scale and equal to the Chernobyl accident.
The agency says the level 7 ranking was made after the damaged reactors have been releasing large amounts of radioactive substances that pose a threat to humans and the environment in a much wider area.
"Level 5 is Three Mile Island. Level 7 is Chernobyl. This doesn't mean it's as bad as Chernobyl. But that's the last time I think 7 was used. It's the highest set by the IAEA (International Atomic Energy Agency)," said physicist Michio Kaku.
Hidehiko Nishiyama with the Nuclear and Industrial Safety Agency (NISA) said the radiation from Fukushima equals about 10 percent of what was released after the Chernobyl accident in 1986.
Nishyama said that the Fukushima event is different from Chernobyl, where a nuclear reactor in Ukraine exploded and sent massive amounts of radiation into atmosphere.
"In the case of Chernobyl, there were many acute radiation injuries reported, 29 people died and in Fukushima's case we have not seen such a situation arise yet," he said.
Operator Tokyo Electric Power Company (TEPCO) has tried to stabilize the nuclear reactor since the magnitude 9.0 quake and tsunami damaged the plant March 11.
Earlier, the Nuclear and Industrial Safety Agency ranked the severity of the Fukushima situation to level 5 on March 18.
At a news conference Tuesday, Nishiyama said after reviewing the radiation amount released since the initial assessment of radioactive iodine-131 and cesium-137, officials decided that the incident should be categorized as a level 7.
"The announcement is being made now because it became possible to look at and check the accumulated data assessed in two different ways," Nishiyama said, in reference to measurements from NISA and the Nuclear Security Council, the Associated Press reported.
The new categorization comes after TEPCO workers put out a small fire at a switchbox that contained batteries in the complex early Tuesday morning, according to a news release.
It was unclear whether the blaze was sparked by a 6.3 aftershock that hit about 48 miles off shore from Tokyo Tuesday.
"The fire was extinguished immediately. It has no impact on Unit 4's cooling operations for the spent fuel rods," said TEPCO spokesman Naoki Tsunoda told the Associated Press.
Officials have been trying to repair the Fukushima plant reactor's cooling system which was damaged after last month's quake.
Experts said the Fukushima crisis will likely not be resolved anytime soon.
"In the best case, we're looking at this plant continuing to leak radiation for months, perhaps, years. It's going to be a long, expensive decommissioning of the plant and a cleanup of the plant," ABC nuclear consultant Joe Cirincione told ABC News Radio.
TEPCO workers were briefly evacuated Monday but reported that there are no damage or irregularities at the complex.
Last week, the same region was rocked by a 7.1 quake that which closed two Japanese nuclear plants and triggered a tsunami warning.
On Monday, officials said people living in five areas 20 miles from Fukushima should leave within a month, the Associated Press reported. Earlier, residents within 12 miles of the plant have been told to evacuate.
In a televised speech on Tuesday, Prime Minister Naoto Kan told people to take things day by day and are not at the point to "let our guards down."
"Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step. The amount of radiation leaks is on the decline," Kan said.
ABC News' Akiko Fujita and the Associated Press contributed to this report.