Japanese nuclear regulators said the rating was being raised from 5 to 7 the highest level on an international scale overseen by the International Atomic Energy Agency after new assessments of radiation leaks from the Fukushima Dai-ichi plant since it was disabled by the March 11 tsunami.
The new ranking signifies a "major accident" with "wider consequences" than the previous level, including widespread health effects, according to the Vienna-based IAEA.
However, Japanese officials have played down any health effects of radioactive releases so far from the Fukushima plant. They said the leaks amount to only a tenth of the radiation emitted in the Chernobyl disaster, while acknowledging they eventually could exceed Chernobyl's emissions if the crisis continues.
"This reconfirms that this is an extremely major disaster. We are very sorry to the public, people living near the nuclear complex and the international community for causing such a serious accident," said Chief Cabinet Secretary Yuki Edano.
But Edano told reporters that there was so far no "direct health damage" from the crisis. "The accident itself is really serious, but we have set our priority so as not to cause health damage."
The revision came a day after the government added five communities to a list of places people should leave to avoid long-term radiation exposure. A 12-mile (20-kilometer) radius already had been cleared around the plant.
Officials from Japan's Nuclear and Industrial Safety Agency said that the cumulative amount of radioactive particles released into the atmosphere since the incident had reached levels that apply to a Level 7 incident. Other factors included damage to the plant's buildings and accumulated radiation levels for its workers.
"We have upgraded the severity level to 7 as the impact of radiation leaks has been widespread from the air, vegetables, tap water to the ocean," NISA official Minoru Oogoda said.
The revision was based on cross-checking and assessments of data on leaks of radioactive iodine-131 and cesium-137, NISA spokesman Hidehiko Nishiyama said.
"We have refrained from making announcements until we have reliable data," Nishiyama said. "The announcement is being made now because it became possible to look at and check the accumulated data assessed in two different ways," he said, referring to measurements from NISA and Japan's Nuclear Security Council.
Nishiyama noted that unlike in Chernobyl there have been no explosions of reactor cores at the Fukushima Dai-ichi plant, although there were hydrogen explosions.
"In that sense, this situation is totally different from Chernobyl," he said.
He said the amount of radiation leaking from the Fukushima Dai-ichi nuclear plant was around 10 percent of the Chernobyl accident.
However, Tokyo Electric Power Co., the operator of the plant, is still estimating the total amount of radioactive material that might be released by the accident, said company spokesman Junichi Matsumoto.
He acknowledged that, if leaks continue, the amount of radioactivity released might eventually exceed the amount emitted by Chernobyl.
The company, under fire for its handling of the accident and its disaster preparedness before the March 11 earthquake and tsunami, issued yet another apology Tuesday.
"We humbly accept this. We deeply apologize for causing tremendous trouble to those who live near the nuclear complex and people in the prefecture," TEPCO spokesman Naoki Tsunoda said.
In Chernobyl, in the Ukraine, a reactor exploded on April 26, 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 19 miles (30 kilometers) around the plant was declared uninhabitable, although some plant workers still live there for short periods and a few hundred other people have returned despite government encouragement to stay away.
In 2005, the Chernobyl Forum a group comprising the International Atomic Energy Agency and several other U.N. groups said fewer than 50 deaths could be confirmed as being connected to Chernobyl. It also said the number of radiation-related deaths among the 600,000 people who helped deal with the aftermath of the accident would ultimately be around 4,000.
The U.N. health agency, however, has said about 9,300 people are likely to die of cancers caused by radiation. Some groups, including Greenpeace, have put the numbers 10 times higher.
Setbacks have continued, meanwhile, at the Fukushima complex, with workers discovering a small fire in a box containing batteries near a reactor building Tuesday. The fire was extinguished quickly, TEPCO said.
It wasn't clear whether the fire was related to a magnitude-6.3 earthquake that shook the Tokyo area Tuesday morning. The cause of the fire was being investigated.
The plant was damaged in a massive tsunami March 11 that knocked out cooling systems and backup diesel generators, leading to explosions at three reactors and a fire at a fourth that was undergoing regular maintenance and was empty of fuel.
The magnitude-9.0 earthquake that caused the tsunami immediately stopped the three reactors, but overheated cores and a lack of cooling functions led to further damage.
Engineers have pumped water into the damaged reactors to cool them down, but leaks have resulted in the pooling of tons of contaminated, radioactive water that has prevented workers from conducting further repairs.
A month after the disaster, more than 145,000 people are still living in shelters. The quake and tsunami are believed to have killed more than 25,000 people, but many of those bodies were swept out to sea and more than half of those feared dead are still listed as missing.
although the threat level now is at the highest level, it is sstill possible things get much worse.
the threat level ranking may need to be re-evaluated to take into account a multiple meltdown situation.