Japan admits nuclear problem is 'severe'

"This is a severe incident that is occurring right now," the spokesman said at a news conference.

"We have vented and used seawater as cooling, followed the accident management plan but this is a very severe operation."

The admission comes as plant operator Tokyo Electric Power Co (Tepco) continues attempts to stop the six-reactor Fukushima 1 complex from going into nuclear meltdown.

"We have to keep cooling the fuel so it doesn't reach criticality," the Tepco spokesman said, adding that radiation levels have barely fallen at the site.

The UK's chief scientific officer John Beddington explained that spent nuclear rods were stored in 'ponds', which kept them cool.

"The situation has changed," he said.

"The pond in rector four is the cause of very considerable concern. What has happened is that this has been damaged by explosions and is leaking very fast.

"We've had reports that it has gone dry."

Low concentrations of radioactive particles from the power plant have been heading eastwards and are expected to reach North America in days, a Swedish official said.

Lars-Erik De Geer, research director at the government-run Swedish Defence Research Agency, cited data gathered from a network of international monitoring stations used to detect nuclear weapons tests.

Meanwhile, international energy authorities and other nations voiced concerns over the situation at the Fukushima plant north-east of Tokyo.

Japanese Chinook helicopters - reportedly fitted with lead radiation shields - attempted to dump tons of seawater into cooling pools to prevent spent fuel overheating while operator working in short shifts pumped water into the reactor cores.

The International Atomic Energy Association (IAEA) said that four water drops were also made after midnight in an attempt to prevent the reactor overheating.

A police riot control water cannon attempted to replenish the cooling pools but was withdrawn, while two military airport fire trucks continued afterwards.

Sebastian Pflugbeil, president of the private German-based Society for Radiation Protection, said Japan's efforts to pull the Fukushima 1 plant back from the brink signalled "the beginning of the catastrophic phase".

"Maybe we have to pray," he said.

The head of the US Nuclear Regulatory Commission, Gregory Jaczko, said he believes the situation is more serious than the Japanese government is letting on.

Jaczko warned water in reactor 4's cooling pool may have run dry and a second reactor could be leaking - something experts say could accelerate the release of radiation.

"We believe that around the reactor site there are high levels of radiation," he said.

"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."

The IAEA said that the reactor 4 cooling pool has trebled in temperature while those for reactors 5 and 6 have more than doubled.

But Japan's nuclear safety agency and Tepco denied Mr Jaczko's statement that the water is gone from the cooling pool.

Utility spokesman Hajime Motojuku said reactor 4's condition is "stable".

Four people were injured in the blast at reactor 1 on March 12, and at least 11 others were hurt in a subsequent explosion at reactor 3 on March 14.

According to the IAEA, two people are still missing and at least 20 workers, police and firemen have suffered radiation contamination at the coastal power plant.

Japan's Meteorological Agency has also revealed that the 9.0 earthquake on March 11 caused parts of the region to sink, heightening the risk of flooding during the next spring tides between March 18 to 26.

Meanwhile, thousands of people have been cleared from their homes, and thousands more have chosen to move away from the region surrounding the Fukushima plant.

Both the UK and US governments have warned citizens living within 50 miles (80 km) of the reactors to evacuate, however recent reports indicate high radiation levels may have been detected 18 miles (30 km) from the stricken plant. 

Japan said the US would fly a high-altitude drone over the stricken complex to gauge the situation with infrared cameras and other monitoring devices.

The rapidly-changing situation has also led Britain, Germany, France, Austria and Australia to urge its citizens to leave the country's capital.

The UK government is chartering flights from Tokyo to Hong Kong to supplement commercially available options for those wishing to leave.

The Foreign Office has advised against any non-essential travel to Tokyo or the quake-hit area.

Asian and European airlines have begun diverting Tokyo flights to Osaka or cancelling them altogether.

The IAEA director general was en route to Japan to assess what he called a "very serious" situation.

Yukiya Amano, the head of the UN atomic watchdog, said he wanted to visit the devastated nuclear plant in Fukushima.

Mr Amano said he would ask Japanese authorities to improve communications with the Vienna-based IAEA, which has been struggling to keep up with fast-developing events because of a lack of timely and detailed data from Japan.

China has urged Japan to give "accurate" and swift information on the nuclear crisis.

Worries about radiation has spurred panic buying of salt in many parts of China on the false belief that it wards off radiation or that future supplies would be contaminated.

The US-based Associated Press has also revealed that behind the escalating nuclear crisis sits a scandal-ridden energy industry in a comfy relationship with government regulators often willing to overlook safety lapses.

Japan's trade minister has also warned that Tokyo may suffer rolling power blackouts if energy demands rise too high.

The power outage comes as a bank network of more than 5,000 cash machines crashed for three hours on Thursday, further unsettling twitchy Japanese citizens.

Meanwhile finance ministers of the leading G7 economies will discuss how to help, as the Bank of Japan injected £50bn into the economy amid a continued stock market slide.

The nuclear crisis has diverted attention from the tens of thousands affected by the quake and subsequent tsunami.

Around 850,000 households in the north are without electricity in near-freezing weather.

The official toll from the twin disasters now exceeds 14,650, police said. The number of confirmed dead is 5,178, with more than 80,000 buildings damaged and 4,798 destroyed.