The move came after Japanese military helicopters dumped water from huge buckets on to the stricken Fukushima nuclear power plant in a bid to douse radioactive fuel rods. The damaged No. 4 unit of the Fukushima Daiichi nuclear complex. White smoke is billowing from the No. 3 unit. Credit:Reuters Four twin-rotor CH-47 Chinooks of the Self-Defence Forces each emptied large buckets that each hold more than seven tonnes of water on to reactors three and four, public broadcaster NHK said. The operation aims to keep the hot fuel rods underwater, to prevent a release of radioactivity when they are exposed to air. Police water cannon were also set to support the effort today, along with firefighting equipment already in use.

The helicopter mission started under a clear sky a day after a first effort was called off by officials citing strong radiation and high winds, shortly before darkness fell. The move came after a US agency helping Japan tackle the crisis said there was no water in one of the reactors' pools, leading to "extremely high" radiation levels, a claim that Japanese officials deny. Instead, Japan's nuclear safety agency said radioactivity levels are continuing to decline at the plant, Reuters reported. A level of 752 microsieverts per hour was recorded at the plant's main gate at 5pm (0800 GMT) on Wednesday, said Tokyo Electric Power Company (TEPCO) official Tetsuo Ohmura. The monitoring point was then changed to the plant's west gate and readings were taken every 30 minutes, he said. At 5am the reading was 338 microsieverts per hour.

We believe that secondary containment has been destroyed and there is no water in the spent fuel pool. And we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures That level was still much higher then it should be, but was not dangerous, Mr Ohmura said. The announcement by the chairman of the US Nuclear Regulatory Commission (NRC), Gregory Jaczko, came as the operator of the Fukushima Daiichi plant said it had almost completed a new power line that could restore electricity to the facility and avoid a meltdown. TEPCO spokesman Naoki Tsunoda said today that officials were planning to test out the line "as soon as possible", but gave no specific time. The line would activate electric-powered pumps, allowing TEPCO to feed a steady water supply to troubled reactors and spent fuel storage ponds so they can be kept cool.

Workers at the plant have been manually feeding seawater into the reactors to prevent the fuel rods from heating up and melting down after Friday's earthquake and tsunami shut down all the country’s nuclear power plants. Mr Jaczko, who was briefing US politicians in Washington, said the NRC believed "there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the spent fuel pool". "We believe that secondary containment has been destroyed and there is no water in the spent fuel pool. And we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures." Mr Jaczko told journalists later that he received the information "from staff people in Tokyo who are interfacing with their Japanese counterparts". "I've confirmed that their information is reliable," he said, adding: "It is my great hope that the information is not accurate."

The NRC and US Department of Energy both have experts on site at the Fukushima Daiichi plant, the Associated Press reported. But Hajime Motojuku, a spokesman for TEPCO, played down the US concerns and said the "condition is stable" at reactor No.4. The US Energy Secretary, Steven Chu, also expressed his concern at the US Energy Funds hearing, saying that "the events unfolding in the Japan incidents actually appear to be more serious than Three Mile Island". "To what extent we don't really know now," he added. High radiation levels prevented the Japanese military from using a helicopter to drop water into reactor No.3 to cool its fuel rods. An earlier explosion had damaged its roof and cooling system. Officials from TEPCO said they would ask the military to try again today.

Bulldozers were also attempting to clear a route to the plant so that fire trucks could try to cool the reactors with hoses. The US's wider exclusion zone The US has designated a much wider no-go zone than the 20-kilometre radius perimeter set up by Japan, after US officials reviewed scientific and technical data.



"We are recommending, as a precaution, that American citizens who live within 50 miles [80 kilometres] of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical," the US embassy in Tokyo said in a statement. The White House admitted the advice it was giving to its citizens went beyond the advice Japan had given, but declined to evaluate the Japanese data.



"This crisis is happening in Japan. Obviously, the Japanese government has the lead here with regards to dealing with the crisis and advising its people," White House spokesman Jay Carney said.



"There are a lot of Americans in Japan. When the data that we receive is analysed independently, we are going to give advice based on our analysis." Very serious, but not out of control: IAEA

The head of the UN's nuclear watchdog, the International Atomic Energy Agency, said he was flying to Japan to assess the situation for himself. Yukiya Amano said at the organisation's daily briefing that the situation at the plant was "very serious".



"Damage to the cores of the three units, units 1, 2 and 3 [of the plant] has been confirmed. The cores remain uncovered by one or two metres," Mr Amano said. But Mr Amano refuted comments by the European Union's energy commissioner Guenther Oettinger that the crisis had spun out of control. "It is not the time to say things are out of control," he said.



The head of the IAEA's department of nuclear safety and security, Denis Flory, agreed.



"We are not talking out of control," Mr Flory said. "The operator, the Japanese government, are doing their utmost to cool down the spent fuel pools and the reactors. This is the best which can be done. They are doing it now, they've been doing it from the beginning and they will continue to do it. This is what I call being in charge of the operation." Growing humanitarian crisis

The nuclear crisis has overshadowed the mounting humanitarian disaster, where poor weather conditions are adding to the woes of half a million homeless survivors and a million more without basic services, food supplies and telecommunications. Survivors and rescue crews had their troubles compounded by snow flurries over roads and rubble.



Temperatures in the worst-hit Iwate, Miyagi and Fukushima prefectures fell sharply to near zero degrees and the meteorological agency forecast a drop to -5 degrees today. Supplies of water and heating oil are low at evacuation centres, where many survivors wait bundled in blankets.



"It's cold today so many people have fallen ill, getting diarrhoea and other symptoms," said Takanori Watanabe, a Red Cross doctor in Otsuchi, a low-lying town where more than half the 17,000 residents are still missing. Japan's National Police Agency said the death toll now stood at 4277, with the total number of dead and missing at 12,000. "The number of deaths stood at 2207 in Miyagi Prefecture, the highest figure among the 12 prefectures where deaths have been reported," the agency said in a statement.

"In Iwate Prefecture, 1482 were killed. In Fukushima Prefecture, the death toll reached 533. "Only 40 [per cent] of the victims had been identified as of Wednesday morning. At least 8194 people are missing in six prefectures. The number stood at 3318 in Iwate, 2507 in Fukushima and 2362 in Miyagi." Australians urged to leave

Fifty-five Australians believed to be in areas worst-affected by the tsunami remain missing today, according to the Department of Foreign Affairs and Trade (DFAT). "Communications remain difficult, and we are continuing to try to confirm their safety. We also urge anyone who has heard from their family or friends in Japan to contact DFAT so we can confirm they are safe," a spokeswoman said.

More than 3700 Australians, including 163 in quake and tsunami-affected areas, are safe, officials in devastated areas have confirmed. Almost 4090 Australians have registered with officials. Foreign Affairs Minister Kevin Rudd said last night that all Australians in Tokyo and eight affected prefectures should consider leaving those locations unless it was essential for them to be there. "That applies to Australians working in the broad business community, the social sector, the education sector or whatever, as well as to our embassy staff as well to whom we are providing the same opportunity, because the embassy, by definition, is in Tokyo," he said on ABC TV program Lateline. When asked about whether Australia would evacuate its citizens, a move France is considering, Mr Rudd said Australia was "workshopping this hour by hour, sometimes minute by minute" with the US, Britain, Canada and New Zealand. The Australian government has again updated its travel advice, adding that people should reconsider travel to Tokyo or surrounding districts due to the risk of disruptions to essential services such as transport and electricity.

However, the DFAT spokeswoman said this was not due to nuclear radiation fears but "the continuing disruption to major infrastructure, its impact on the welfare of people on the ground and continuing aftershocks". Emperor's address A rare television appearance by Japanese Emperor Akihito last night underlined the severity of the situation his country faces. Emperor Akihito said he was "deeply concerned" about the "unpredictable" situation at the Fukushima Daiichi power plant. "I sincerely hope that we can keep the situation from getting worse," Emperor Akihito said in a historic address that marked the first time he has intervened in a national crisis.

Estimates of losses to Japanese output from damage to buildings, production and consumer activity ranged from between 10 and 16 trillion yen ($127-$204 billion), up to 1½ times the economic losses from the devastating 1995 Kobe earthquake, Reuters reported. - with Megan Levy and agencies Loading Follow Environment on Twitter

