Japan has suspended operations to prevent quake-affected Fukushima nuclear plant from melting down after a surge in radiation made it too dangerous for workers to remain at the facility.
Yukio Edano, Japan's chief cabinet secretary, said on Wednesday work on dousing reactors with water was disrupted by the need to withdraw.
The level of radiation at the plant surged to 1,000 millisieverts early on Wednesday before coming down to 800-600 millisieverts. Still, that was far more than the average. "So the workers cannot carry out even minimal work at the plant now," Edano said.
Earlier, a fire broke out at the nuclear reactor again, a day after the Fukushima Daiichi power plant emitted a burst of radiation that panicked an already nervous Japan and left the government struggling to contain a crisis caused by last week's earthquake and tsunami.
The outer housing of the containment vessel at the Unit 4 at the nuclear complex erupted in flames early on Wednesday, Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Co (TEPCO), said.
On Tuesday, a fire broke out in the same reactor's fuel storage pond - an area where used nuclear fuel is kept cool - causing radioactivity to be released into the atmosphere.
TEPCO said the new blaze erupted because the initial fire had not been fully extinguished.
About three hours after the blaze erupted on Wednesday, Japan's Nuclear and Industrial Safety Agency said fire and smoke could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out.
Public broadcaster NHK also said flames were no longer visible at the building housing the Unit 4 reactor.
The Nuclear and Industrial Safety Agency said two workers were missing after the multiple explosions and fires at the plant in Fukushima, 240km north of Tokyo, since the quake and tsunami damaged its cooling system.
Radiation levels in areas around the nuclear plant rose early on Tuesday afternoon but appeared to subside by evening, officials said.
But the unease remained from the massive disasters that are believed to have killed more than 10,000 people and rattled the world's third-largest economy.
The radiation leak caused the government to order 140,000 people living within 30km of the plant to seal themselves indoors to avoid exposure, and authorities declared a ban on commercial air traffic through the area.
'Fuel rods damaged'
Separately, the Nuclear and Industrial Safety Agency said 70 per cent of the nuclear fuel rods in Unit 1 may have been damaged following an explosion.
Minoru Ohgoda, a spokesman, said "it's likely that roughly about 70 per cent of the fuel rods may be damaged".
But he said "we don't know the nature of the damage, and it could be either melting, or there might be some holes in them".
Worries about radiation hung heavy over Tokyo and other areas far beyond that cordon. The stock market plunged for a second day, dropping 10 per cent.
The troubles cascaded on Tuesday at the Daiichi plant, where there have already been explosions at two reactor buildings since Friday's disasters.
An explosion at a third reactor tore out a 26ft hole in the building and, experts said, damaged a vessel below the reactor, although not the reactor core. Three hours later, a fire broke out at a fourth reactor, which had been offline for maintenance.
In a nationally televised address on Tuesday, Naoto Kan, the Japanese prime minister, said radiation had seeped from four of the plant's six reactors.
The International Atomic Energy Agency said Japanese officials informed it that the fire was in a pool where used nuclear fuel rods are stored and that "radioactivity is being released directly into the atmosphere".
Long after the fire was extinguished, a Japanese official said the pool might still be boiling. Depending on how bad the blast was at Unit 2, experts said more radioactive materials could seep out.
If the water in the storage pond in Unit 4 boils away, the fuel rods could be exposed, leaking more virulent radiation.
Experts noted that much of the leaking radiation was apparently in steam from boiling water and the falling radiation levels suggest the situation could be stabilising.
Al Jazeera’s Steve Chao, reporting from Yamagata on Tuesday, said a no-fly zone had been established in a 30km radius over the Fukushima plant.
Four out of six reactors at the facility were in trouble, he said.
Yukio Edano, the government spokesman, said the radiation leak potentially affected public health. But authorities and experts said the risks to the public diminished the farther the distance from the plant.
At its most intense, the leak released a radioactive dose in one hour at the site 400 times the amount a person normally receives in a year. Within six hours, that level had dropped dramatically.
A person would have to be exposed to that dose for 10 hours for it to be fatal, Jae Moo-sung, a nuclear engineering expert at Seoul's Hanyang University, said.
Radiation elsewhere never reached that level. In Tokyo, 270km to the southwest, authorities reported radiation levels nine times normal - too small, officials said, to threaten the 39 million people in and around the capital.
Weather patterns helped, shifting on Tuesday night to the southeast, blowing any potential radiation from the plant towards the sea.
The IAEA said on Tuesday that all other Japanese nuclear plants were in a safe and stable condition.
Though Kan and other officials urged calm, the developments triggered panic in Japan and around the world amid widespread uncertainty over what would happen next.
In the worst case scenario, one or more of the reactor cores would completely melt down, a disaster that could spew large amounts of radioactivity into the atmosphere.
Three of the plant's six reactors were out of service for maintenance at the time of Friday's disasters, which compromised cooling systems at all of the reactors. Before Tuesday's fire in Unit 4's storage pool, workers were desperately trying to pump seawater to cool the fuel rods in the three active reactors.
Conditions in Unit 2 are less clear after a blast near a suppression pool, into which fuel rods are plunged to cool them and which also serves as an emergency receptical for excess steam, according to TEPCO.
The nuclear core was not damaged but the bottom of the surrounding container may have been, Shigekazu Omukai, a spokesman for Japan's nuclear safety agency, said.
The IAEA's head, Yukiya Amano, urged the Japanese government to provide better information to the agency about the situation.
Temperatures in the two other offline reactors, units 5 and 6, were slightly elevated, Edano, who is also Japan's chief cabinet secretary, said.
Fourteen pumps have been brought in to get seawater into the other reactors, and technicians were trying to figure out how to pump water into Unit 4, where the storage pool fire occurred.
Early on Wednesday, TEPCO officials said they had scrapped a plan to use helicopters, deeming them impractical, and said they were considering other options, including using fire engines.
About 70 workers remained at the complex, struggling with its myriad problems. The workers, all in protective gear, are being rotated in and out of the danger zone quickly to reduce their radiation exposure.
Kan and other officials warned 30km of the Fukushima plant to stay indoors to avoid exposure that could make people sick.
Some 70,000 people had already been evacuated from a 20km radius from the Daiichi complex. About 140,000 remain in the wider zone.
Four days after the 9.0-magnitude earthquake and tsunami, millions of people strung out along the east coast had little food, water or heat, and already chilly temperatures dropped further as a cold front moved in. Up to 450,000 people are in temporary shelters.
Likely death toll
Officials have only confirmed about 3,300 deaths, but officials have said the toll was likely to exceed 10,000 in one of the four hardest-hit areas. Experts involved in the 2004 Asian tsunami said there was no question more people died, despite Japan's high state of preparation, and like the earlier disaster, many thousands may never be found.
In a rare bit of good news, rescuers found two survivors on Tuesday, one of them a 70-year-old woman whose house was torn off its foundation by the tsunami.
Mostly, though, search teams found few signs of life. More than 200 rescue crews from the US and Britain poured on Tuesday into the coastal city of Ofunato, finding little but rubble and people looking for lost possessions.
As rescue teams and survivors hunted through ruined communities and officials struggled to deliver supplies to the displaced, Japan was shaken by more strong aftershocks, prompting buildings to sway in Tokyo.
The first, measuring 6.2 in magnitude, struck on Tuesday night off the coast of Fukushima prefecture.
Three minutes later, a 6.0-magnitude quake hit Shizuoka prefecture, 90km southwest of Tokyo.
"We had an aftershock of about 6-magnitude," Al Jazeera's Wayne Hay, reporting from Akita, said. "I was on the 11th story and certainly the building did sway for about 30 seconds."
Foreigners began leaving in larger numbers on Tuesday. China organised an evacuation of its citizens from Japan's stricken northeast. The US urged Americans to avoid travel to Japan. Austria moved its embassy from Tokyo to Osaka. Lufthansa diverted its two daily flights to Tokyo to other Japanese cities.
The US navy shifted some ships from Japan's east coast to western waters to avoid hazards from debris dragged into the sea by the tsunami and to be away from any radiation plume. One ship at its base south of Tokyo detected low levels of radiation from the Fukushima plant.