Japan's nuclear watchdog has raised the severity level of the crisis at its stricken nuclear power plant to 7 - the highest level and equal to the disaster at Chernobyl in the former Soviet Union.
The incident at the Fukushima Daiichi plant had previously been rated a 5 on an international scale used to gauge the severity of incidents at nuclear facilities.
But Japan's Nuclear and Industrial Safety Agency said the amount of discharged radioactive materials is approximately 10 per cent of the amount released when a nuclear reactor exploded at the Chernobyl plant in 1986.
The statement was released as a strong earthquake with a preliminary magnitude of 6.3 jolted the Tokyo area and its surrounding areas. Japan's Meteorological Agency said the quake struck at 8:08am local time [23:08 GMT] the same day.
A powerful aftershock recorded on Monday evening in the Fukushima prefecture killed three people, emergency workers said.
No casualties were reported in Tuesday's quake, and there were no reports of damage in the Tokyo prefecture, nor any tsunami warning issued.
But the Tokyo Electric Power Co (TEPCO), which operates the Fukushima Daiichi nuclear complex, said a fire had briefly broken out at the Number 4 reactor.
TEPCO said the fire at a box that contained batteries in a building near the reactor was discovered at about 6:38am and put out seven minutes later. It was not clear whether the fire was related to Tuesday morning's earthquake. The cause was being investigated.
Earlier, the government expanded an evacuation zone around the Fukushima plant because of the high levels of accumulated radiation since a tsunami hit the complex a month ago, causing massive damage to its reactors.
Two additional quakes early afternoon on Tuesday prompted TEPCO to once again order workers to evacuate the nuclear plant, bringing the total number of earthquakes above magnitude 6 in the region to four in roughly 24 hours.
A company spokesman told Reuters news agency that the status of the damaged plant is being inspected, but did not provide additional details.
Al Jazeera's Harry Fawcett, reporting from Mizusawa on Tuesday, said there is a lot of concern among Japanese people about the long-term impact of the radiation in the areas surrounding the site.
"Certainly the refugees, or the people who've been evacuated ... they've often being saying to us they're not sure they're being told the full story," he said. "The impact is certainly spreading."
Local news agency Kyodo said the government's Nuclear Safety Commission had estimated that at one stage the amount of radioactive material released from the reactors in northern Japan had reached 10,000 terabequerels per hour for several hours, which would classify the incident as a major accident according to the INES scale.
The International Nuclear and Radiological Event Scale (INES), published by the International Atomic Energy Agency, ranks nuclear incidents by severity from 1 to a maximum of 7.
Kyodo did not say when the big increase in radiation had happened but quoted the commission as saying the release had since fallen to under one terabecquerel per hour.
The commission also released a preliminary calculation for the cumulative amount of external exposure to radiation, saying it exceeded the yearly limit of one millisieverts in areas extending more than 60km to the northwest of the plant and about 40km to the south-southwest.
Jasmina Vujic, professor of nuclear engineering at University of California, Berkeley, said that there were major differences between the Chernobyl and Fukoshima disasters.
Chernobyl was situated in an area with a high population density in the centre of Europe, she said, while the Japanese plant is in a lightly populated, coastal region.
"From that point of view, the impact on environment and population, would be much smaller than Chernobyl," Vujic told Al Jazeera.
Japan had previously assessed the accident at reactors operated by TEPCO at level 5, the same level as the Three Mile Island accident in the US in 1979.
The tsunami was triggered by a 9.0 magnitude earthquake on March 11, the largest recorded in quake-prone Japan, crippling the reactors' cooling systems.
TEPCO said on Monday it had stopped the discharges of low-level radioactive water into the sea that have drawn complaints from neighbouring China and South Korea.
It has already pumped 10,400 tonnes of low-level radioactive water into the ocean to free up storage capacity for highly contaminated water from the reactors.
On Monday, shortly after Japan marked one month since the quake, a huge aftershock shook a wide swathe of eastern Japan, killing two people, and knocking out power to 220,000 homes.
It was one of more than 400 aftershocks above a 5 magnitude to have hit the area since March 11.
Because of accumulated radiation contamination, the government is encouraging people to leave certain areas beyond its 20km exclusion zone around the plant. Thousands of people could be affected by the move.
Masataka Shimizu, TEPCO's president, visited the area on Monday for the first time since the disaster. He had all but vanished from public view apart from a brief apology shortly after the crisis began and has spent some of the time since in hospital.
Source: Al Jazeera and agencies