Japan's Emperor Akihito has said he is "deeply worried" about the crisis his country is facing following last Friday's earthquake and tsunami.
In an extremely rare appearance, the emperor went on live TV to make his first public comments on the disaster, and urged an all-out rescue effort.
He spoke after technicians temporarily abandoned a quake-crippled nuclear plant as radiation briefly surged.
Thousands of people were killed in the 9.0-magnitude quake and tsunami.
The latest spike of radiation measured at Fukushima Daiichi is consistent with reports of damage to the containment system around reactor 3.
It appears similar to Tuesday's incident in reactor 2, thought to be a crack that released steam containing some radioactive substances.
In both cases, radioactivity levels soon fell. More worryingly, the latest spike was serious enough to force technicians off the site.
Without workers on the ground, it is hard to see how the vital task of putting water into the hot reactors could be maintained.
Reactor 4's situation is puzzling. A pool storing fuel rods from the shut-down reactor appears to have run dry, and officials spoke of a danger of "criticality" - the resumption of a nuclear reaction in the stored rods.
Nuclear experts say this should not happen if the rods have been correctly arranged in the pool.
Radioactivity levels continue to be an issue around the plant; but further away, they are not high enough at present to pose a hazard - especially with the wind blowing out to sea.
The stricken Fukushima Daiichi power facility has sent low levels of radiation wafting into Tokyo, spreading alarm in the city and internationally.
TV stations interrupted programming on Wednesday to show the emperor describing the crisis facing the nation as "unprecedented in scale".
The 77-year-old - deeply respected by many Japanese - said: "I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times."
Japan's titular head of state - who acceded to the throne in 1989 after the death of his father Hirohito - said he prayed that every victim would be saved.
He spoke as snow blanketed swathes of the disaster zone, where many survivors have little food, water or heat.
About 450,000 people have been staying in temporary shelters, many sleeping on the floor of school gymnasiums.
Nearly 3,700 people are listed as dead, but it is feared the total death toll will top 10,000 following the catastrophe, which pulverised the country's north-east coast.
Helicopters deployed to dump water on the facility on Wednesday have been pulled out amid concerns over radiation.
Earlier, the plant's operators evacuated its skeleton crew of 50 workers for about an hour as ground-level radiation spiked.
And yet another fire broke out in a reactor, while smoke or steam billowed from another one.
Winds from the facility are currently blowing north-west out into the Pacific Ocean.
Levels of radiation outside the plant have now fallen from 1,000 millisieverts an hour to 600-800. A single dose of 1,000 millisieverts can cause temporary radiation sickness.
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Fukushima Daiichi crisis
* Reactor 1: Was first to be rocked an explosion on Saturday; fuel rods reportedly 70% damaged
* Reactor 2: There are fears a blast on Tuesday breached a containment system; fuel rods reportedly 33% damaged
* Reactor 3: Explosion on Monday; smoke or steam seen rising on Wednesday; damage to roof and possibly also to a containment system
* Reactor 4: Hit by a major blaze on Tuesday and another fire on Wednesday
* In graphics: Fukushima alert
The governor of Fukushima prefecture, Yuhei Sato, criticised official handling of the crisis, telling Japan's NHK TV: "The anxiety and anger being felt by people in Fukushima have reached a boiling point."
Tokyo Electric Power Co, which runs the plant, said three-hour power cuts on Wednesday would affect nearly 11 million households.
The government said it had no plans to extend a 20-km (12-mile) evacuation zone around the facility.
Another 140,000 people living between 20-30km of the facility were told on Tuesday not to leave their homes.
The atomic crisis has been caused by the tsunami breaking back-up diesel generators which kept the nuclear fuel cool.
Workers have been dousing the reactors with seawater in a frantic effort to stabilise their temperatures, since the first in a series of explosions rocked the plant on Saturday.
In Tokyo, there were as few people on the streets as there would be on a public holiday, and many shops and offices were shut.
Many people have stocked up on food and water to stay indoors or have simply left the city.