The Japanese government's nuclear safety agency raised the crisis level of the Fukushima Dai-ichi power plant accident from 5 to 7, the worst on the international scale and on par with the Chernobyl accident 25 years ago.
The emission of radioactive substances from the stricken plant is about 10 percent of the amount that had been detected at Chernobyl, the agency said on Tuesday.
However, an official at Tokyo Electric and Power (TEPCO) said later Tuesday that they are concerned that the radiation leakage could eventually exceed the 1986 disaster.
"The radiation leak has not stopped completely and our concern is that it could eventually exceed Chernobyl," a TEPCO official told reporters.
The Kyodo news agency said the agency estimated the amount of radioactive material released from the reactors in northern Japan reached a maximum of 10,000 terabequerels per hour at one point for several hours, which would classify the incident as a major accident, according to the International Nuclear and Radiological Event Scale (INES).
The rating reflects the initial severity of the crisis, not the current situation, which has seen radiation levels drop dramatically.
"This is a preliminary assessment, and is subject to finalization by the International Atomic Energy Agency," said an official at the Nuclear and Industrial Safety Agency (NISA), the government's nuclear watchdog, which made the announcement with the Nuclear Safety Commission.
Japan is struggling to regain control of the plant after a massive earthquake and tsunami devastated its northeast on March 11, and is facing a major humanitarian and economic crisis.
The INES scale is published by the IAEA.
Japan had previously assessed the accident at reactors operated by Tokyo Electric Power Co, at level 5, the same level as the Three Mile Island accident in 1979.
Small fire extinguished
Also Tuesday, workers at Japan's tsunami-stricken nuclear power complex discovered a small fire near a reactor building Tuesday but it was extinguished quickly, the plant's operator said.
TEPCO said the fire at a box that contains batteries in a building near the No. 4 reactor was discovered at about 6:38 a.m. Tuesday and was put out seven minutes later.
It wasn't clear whether the fire was related to a magnitude-6.3 earthquake that shook the Tokyo area Tuesday morning. Kyodo said Japan's main international airport Narita closed runways for checks but later resumed flights. The cause of the fire is being investigated.
"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.
Aftershocks rattle nerves
There have been hundreds of aftershocks since March 11 when a massive 9 magnitude earthquake and 15 meter tsunami hit northeast Japan, plunging the country into its worst crisis since World War Two.
A second major aftershock measuring 6.3 rocked northeast Japan on Tuesday, swaying buildings in central Tokyo, shortly after the government upgraded its nuclear crisis.
The latest aftershock struck the area of Fukushima, near the crippled Dai-ichi nuclear plant, said the Japan Meteorological Agency. A 6.3 aftershock hit Chiba prefecture, neighboring Tokyo, earlier on Tuesday.
An aftershock measuring 6.6 quake hit Fukushima prefecture on Monday evening temporarily cutting power and forcing workers to evacuate the nuclear plant.
In Iwaki, a landslide brought down three houses, trapping up to seven people. Four were rescued alive, but one of those — a 16-year-old girl — died at the hospital, a police official said. He would not give his name, citing policy.
Around 210,000 people have no running water and, following Monday's aftershocks, more than 240,000 people are without electricity.
In all, nearly 190,000 people have fled their homes, the vast majority of whom are living in shelters, according to the national disaster agency. About 85,000 are from the cleared zone around the nuclear plant; their homes may be intact, but it's not known when they'll be able to return to them.
Yutaka Endo said he feels like his life has been put on hold because of the nuclear crisis.
He fled Minami Soma and has been living in a shelter in Fukushima city for three weeks with his family.
"I can't make any plans because of the nuclear crisis. My home was fine, but I can't go back there because it is in a restricted area," said the 32-year-old, who used to tend bar. "I need to find a new job and a place to live so that we can get out of here. But I can't do anything until these zones are lifted."
Ryokou Sasaki said he and his elderly parents are in the same position. They've applied for temporary shelters, and are waiting to hear back.
He recently moved back home — to the northeastern port city of Kamaishi — to help his parents' with their fishing business.
"We're not in a place yet where we can even think about rebuilding the business yet," said the 40-year-old. "They seem to have given up."