|Al Jazeera's Andrew Thomas reports from Tokyo where radiation fears have caused unease.
Workers battling to contain the crisis at Japan's quake-stricken Fukushima nuclear plant were briefly moved to a bunker because of a rise in radiation levels, local media has reported.
The level of radiation at the plant surged to 1,000 millisieverts early on Wednesday before coming down to 800-600 millisieverts.
Harry Fawcett, Al Jazeera's correspondent in Japan, said the workers struggling to avert a nuclear meltdown were allowed to return to the facility later.
"The 70 workers who were taken into that protective bunker were able to go back and restart operations crucial to keeping this entire plant cool," he said.
"They have been pumping sea water into the reactors; the ones that were active before the earthquake and the ones which were just housing spent fuel," he added.
Yukio Edano, Japan's chief cabinet secretary, said the workers dousing the reactors in a frantic effort to cool them needed to be taken to safety after an explosion a day earlier in the complex's Unit 4 reactor led to a surge in radiation.
The blast is thought to have damaged the reactor's suppression chamber, a water-filled pipe outside the nuclear core that is part of the emergency cooling system.
Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Company, said the outer housing of the containment vessel at the Number 4 unit in the plant was in flames on Wednesday.
The plant, 220km north of Tokyo, has been hit by several explosions after a devastating earthquake and tsunami last Friday damaged its cooling fuctions.
Damage to reactors
Broadcaster NHK showed photographs of the reactors 3 and 4 at the plant, showing damage to both.
Later on Wednesday, a Japanese military twin-rotor cargo helicopter started dumping water onto a reactor at the plant in an effort to cool the rods, television images showed.
The Japanese authorities had earlier ruled out using helicopters to pour water into the reactors, saying it was a high risk operation.
The mission, however, was soon abandoned, NHK reported.
"I assume it hasn't worked or levels of radiation are too dangerous to be in the air near those stations," our correspondent said.
"The situation is deteriorating so badly they had to try. They are doing anything that they can."
The government has ordered some 140,000 people in the vicinity to stay indoors.
There are six reactors at the plant. The one still on fire was not in use at the time of the magnitude 9.0 quake, Japan's most powerful on record.
|Water has been dumped on the reactors in an effort to cool overheating rods [Reuters]
The Nuclear and Industrial Safety Agency estimated that 70 per cent of the rods have been damaged at the Number 1 reactor.
Japan's national news agency, Kyodo, said that 33 per cent of the fuel rods at the Number 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.
Also on Wednesday, a strong 6.0 magnitude earthquake was felt across large areas of eastern Japan, the US Geological Survey said, with the force strong enough to sway buildings in the capital Tokyo.
The quake struck in the Pacific off Chiba prefecture - 96km east of the capital. There were no reports of injuries or damage following the quake, which struck at a shallow depth of 25km at 12:52pm (0352 GMT), police and local government officials said.
No tsunami warning was issued but the Japan meteorological agency warned of a possible change in sea levels.
Meanwhile, Japanese emperor Akihito gave a rare television address to the nation, in which he described the catastrophe to have befallen the nation as "unprecedented in scale".
The 77-year-old said he was "deeply worried" about the situation at the crisis-hit Fukushima nuclear plant, in an appearance which interrupted scheduled programming.
"I am deeply hurt by the grievous situation in the affected areas. The number of deceased and missing increases by the day and we cannot know how many victims there will be. My hope is that as many people possible are found safe," Akihito 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," he said, urging survivors "not to abandon hope".
Authorities are staring at a staggering death toll following last week's twin disasters which decimated Japan's northeastern coastline.
Police say at least 5,000 people were killed as huge waves swept ashore, sweeping away everything in its path.
But officials say the death toll could top 10,000 as many more are still unaccounted for.
The devastation in the tsunami-hit areas such as the small fishing town of Minamisanriku have been huge, with the northeastern settlement missing about half of its 17,000 people.
"Ten of my relatives are missing. I haven't been able to get in contact with them," 54-year-old Minamisanriku resident Tomeko Sato, who lost her house in the disaster, told AFP news agency.
"I was very surprised by the power of the tsunami... next time, I will live on the hill and hope it never happens again."
Andrew Thomas, Al Jazeera's correspondent in Osaka, Japan, said the trouble with the tsunami is that many of those people may never be found having been washed out to sea. He also said the weather had taken a turn for the worst for any survivors."
Another of our correspondents in the north said apart from a few isolated incidents of rescue on Tuesday, the large amount of work rescuers do is recovering bodies and looking after those who have been displaced.
Millions in Japan have been left without water, electricity, fuel or enough food. Hundreds of thousands more are homeless, stoically coping with snow and freezing rain in the northeast.
Source: Al Jazeera and agencies