The Japanese head of the International Atomic Energy Agency, the United Nations' nuclear watchdog, has denied that the situation at an earthquake-damaged nuclear plant is out of control.
"It is not the time to say things are out of control," Yukiya Amano told a news conference. "The operators are doing the maximum to restore the safety of the reactor.
Yukiya Amano was responding to comments by Guenther Oettinger, the EU energy chief, suggesting that efforts to contain the crisis at the Fukushima plant had failed after efforts to cool a reactor by dumping water on it from a helicopter were abandoned.
The plant has been hit by a series of explosions since Friday's quake knocked out reactor cooling systems.
"In the coming hours there could be further catastrophic events which could pose a threat to the lives of people on the island," Oettinger told a European Parliament committee.
There was hope early on Thursday that a newly built power line that could restore electricity at the plant would soon be ready. If the plant's electricity-powered pumps can be restarted, water could be pumped to the damaged reactors to cool them.
Naoki Tsunoda, a spokesman for Tokyo Electric Power Company, said officials planned to try the line as soon as possible, but could not say when, the AP news agency reported.
Surging radiation levels forced Japan to order emergency workers at the crippled nuclear plant to be briefly moved to a bunker, as reported in the local media, in the desperate battle to cool the overheating reactors.
Al Jazeera's Harry Fawcett said workers struggling to avert a nuclear meltdown at the Fukushima nuclear complex 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."
Yukio Edano, the Japanese 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 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 Unit 4 was in flames on Wednesday.
The level of radiation at the plant surged to 1,000 millisieverts early on Wednesday before coming down to 800-600 millisieverts.
But Gregory Jaczko, head of the US Nuclear Regulatory Commission, said Wednesday that emergency workers may have been exposed to "lethal doses" of radiation.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time," told a House subcommittee.
Hundreds of thousands of people have been evacuated from a 20 km zone around the plant, while those within 30 km of the plant have been told to stay indoors because of the risk from heightened radiation levels.
The official death toll from Friday's massive earthquake and tsunami rose to 4,340 on Wednesday although thousands of people are still missing and officials say at least 10,000 have likely been killed.
Japanese Emperor Akihito also gave a rare televised address to the nation describing the catastrophe as being of an "unprecedented scale".
Many governments have urged their citizens to leave Tokyo and earthquake-affected areas amid concerns Japan's capital could be exposed to dangerous levels of radiation if the situation worsens.
Great Britain said Britons in Tokyo should leave because of the "evolving situation at the Fukushima nuclear facility". Australia, France and Germany have also advised their citizens to leave Tokyo.
The US said Americans living within 80 km of the plant should leave the area or take shelter indoors.
Tokyo reported slightly elevated radiation levels on Tuesday, but officials said the increase was too small to threaten the 39 million people in and around the capital.
Businesses have issued similar travel advisories telling people to consider moving away from disaster areas if they have no reason to be there.
Meanwhile US forces in Japan have been told to stay at least 80 km from the crippled Fukushima complex without special authorisation.
Reuters news agency, citing the US military, said potassium iodide tablets have been given to some air crews ahead of missions as a precaution against radiation, though no personnel in Japan are showing signs of radiation poisoning.
Authorities are staring at a staggering death toll following last week's twin disasters which decimated Japan's northeastern coastline.
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.