All remaining workers at the Fukushima atomic power plant have abandoned the ailing facility after a fire broke out at a reactor.
A reactor containment vessel plant may have suffered damage as a result of the blaze, the chief government spokesman Yukio Edano said.
Edano told a briefing that the likeliest explanation for a white cloud seen above the plant was that steam came from part of the containment vessel in the number-three reactor.
Kyodo news agency has reported that pressure is stable at the number-three reactor core’s vessel.
There are still 145 Australians unaccounted for after a 9.0-magnitude earthquake and tsunami rocked Japan last Friday.
The pre-dawn blaze at the number-four reactor of the Fukushima No. 1 plant reportedly went out of its own accord 30 minutes after being spotted.
Engineers have been battling a nuclear emergency at the 40-year-old plant since the quake knocked out cooling systems and fuel rods began overheating.
All but 50 nuclear workers, dubbed the "Fukushima 50", had been evacuated from the quake-hit facility amid fears of a meltdown. But these workers, regarded as heroes for their tireless efforts to douse three stricken reactors with sea water, were pulled out after today's fire.
CNN reports that workers have since been allowed to return to the plant.
There have been four explosions and two fires at four of the plant's six reactors, and radioactive material has been released into the atmosphere. Two workers have been missing since the disaster struck.
Radiation levels near the plant had reached levels harmful to health yesterday, the government said.
Tens of thousands have been evacuated from a 20km zone around the plant and thousands of others within a 20-30 km radius were urged to stay indoors.
Eight experts from the US Nuclear Regulatory Commission were to arrive today to advise on managing the situation.
The hydrogen gas blasts which began Saturday have shattered or damaged buildings housing the reactors and a reactor containment vessel may have suffered damage, the chief government spokesman said.
The government has also reported apparent damage to the suppression pool surrounding the base of the containment vessel of the number-two reactor.
The nuclear safety agency, citing information from TEPCO, said 70 percent of the fuel rods at the number-one reactor and 33 percent at the number-two reactor are believed damaged judging by radiation levels.
It was possible the rods' metal cladding had melted, exposing the radioactive core, a spokesman said.
Seawater is being pumped around the fuel rods in a desperate attempt to cool them down, but there are also fears about pools which hold spent rods.
If water in the pools evaporates, the spent rods would be exposed to the air and radioactive material would be released into the atmosphere.
Plant operator Tokyo Electric Power initially considered spraying boracic acid over the containment pool at reactor number four but is now leaning towards pumping it through fire engines, the safety agency spokesman said.
Boracic acid curbs nuclear fission by absorbing neutrons, a key element in a nuclear chain reaction.
The main US nuclear energy regulator backed Japan's efforts, saying Tuesday it had taken appropriate actions. But there was greater concern elsewhere.
Scared Tokyo residents filled outbound trains and rushed to shops to stock up on face masks and emergency supplies amid heightening fears of radiation headed their way.
Officials said Tuesday the levels in the mega-city were above normal but not harmful to health.
France's Nuclear Safety Authority said the disaster now equated to a six on the seven-point international scale for nuclear accidents, ranking the crisis second only in gravity to Chernobyl.
Europe's energy commissioner Guenther Oettinger went further and dubbed the nuclear disaster an "apocalypse", saying Tokyo had almost lost control of events at the Fukushima plant.
"There is talk of an apocalypse and I think the word is particularly well chosen," he said in remarks to the European Parliament.