A new fire on Wednesday at a quake-hit atomic power plant compounded Japan's nuclear crisis as a team of US experts headed to the country to help avert disaster.
The pre-dawn blaze at the number-four reactor of the Fukushima No. 1 atomic power plant reportedly went out of its own accord 30 minutes after being spotted, the state atomic safety agency said.
But white smoke or steam was seen above the stricken plant around 10:00 am (0100 GMT).
Media reports said that it was coming from the reactor number three building, which was hit by a blast on Monday that tore off the building's outer structure.
Plant operator Tokyo Electricity Power Co (TEPCO) said that it did not know where the smoke was coming from.
Engineers have been battling a nuclear emergency at the 40-year-old plant since a massive earthquake and tsunami knocked out cooling systems last Friday and fuel rods began overheating.
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 on Tuesday, the government said.
Tens of thousands have been evacuated from a 20 kilometre (12 mile) 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 on Wednesday to advise on managing the situation, the foreign ministry said.
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% of the fuel rods at the number-one reactor and 33% 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.
TEPCO 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 on 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 on Tuesday that 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.