Japan has raised the alert level at its quake-damaged nuclear plant from four to five on a seven-point international scale of atomic incidents.
The crisis at the Fukushima Daiichi site, previously rated as a local problem, is now regarded as having "wider consequences".The UN says the battle to stabilise the plant is a race against time.
The crisis was prompted by last week's huge quake and tsunami, which has left at least 17,000 people dead or missing.
Japanese nuclear officials said core damage to reactors 2 and 3 had prompted the raising of the severity grade.
The 1979 incident at Three Mile Island in the US was also rated at five on the scale, whereas the 1986 Chernobyl disaster was rated at seven.
Japan's government has conceded it could have moved more quickly in dealing with the nuclear crisis.
Panic-buying Further heavy snowfall has all but ended hopes of rescuing anyone else from the rubble after the 9.0-magnitude quake and tsunami.
Millions of people have been affected by the disaster - many survivors have been left without water, electricity, fuel or enough food; hundreds of thousands are homeless.
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For seven days, workers and emergency personnel at the Fukushima plant have been battling to prevent a disaster turning into a catastrophe.
Radiation is still leaking from the site, and although levels dropped during the day, no-one involved in the operation is suggesting they are winning yet.
At the plant again today they pumped huge amounts of water through fire hoses on to the damaged reactors and the pools holding spent fuel rods to try to cool them.
The International Atomic Energy Agency says the situation remains very serious, but is not getting worse.
Japan's nuclear safety watchdog is now categorising this as an accident with wider consequences - an innocuous phrase that does not really do justice to the disruption that's been caused.
Analysis
Radiation is still leaking from the site, and although levels dropped during the day, no-one involved in the operation is suggesting they are winning yet.
At the plant again today they pumped huge amounts of water through fire hoses on to the damaged reactors and the pools holding spent fuel rods to try to cool them.
The International Atomic Energy Agency says the situation remains very serious, but is not getting worse.
Japan's nuclear safety watchdog is now categorising this as an accident with wider consequences - an innocuous phrase that does not really do justice to the disruption that's been caused.
The national police say 6,911 people are known to have died in the disaster, and 10,316 are still missing.
On Friday, people across Japan observed a minute's silence at 1446 (0546 GMT), exactly one week after the disaster.As the country paused to remember, relief workers toiling in the ruins bowed their heads, and some elderly survivors in evacuation centres wept.
Japanese officials continue to try to reassure people that the radiation risk is virtually nil outside the 30-km (18-mile) exclusion zone around the plant.
But foreign governments are taking wider precautions - Spain has joined Britain, the US and other countries in organising the evacuation of any of their citizens who are concerned.
And panic has spread overseas, with shops in parts of the US being stripped of iodine pills, which can protect against radiation, and Asian airports scanning passengers from Japan for possible contamination.
Shoppers in China have been panic-buying salt in the mistaken belief that it can guard against radiation exposure.
Meanwhile, Prime Minister Naoto Kan told a national television address: "We will rebuild Japan from scratch. We must all share this resolve."
He said the natural disaster and nuclear crisis were a "great test for the Japanese people", but exhorted them all to persevere.
On the nuclear issue, Chief Cabinet Secretary Yukio Edano accepted that "in hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster".
International Atomic Energy Agency chief Yukiya Amano arrived earlier in Tokyo and warned the Fukushima crisis was a "race against the clock".
"This is not something that just Japan should deal with, and people of the entire world should co-operate with Japan and the people in the disaster areas," said Mr Amano, a Japanese citizen.
The International Atomic Energy Agency said its team of experts had taken radiation readings from Tokyo and found nothing to cause concern, saying there was no indication of radioactive iodine or caesium in the city.
The agency announced it would hold a special board meeting on Monday to discuss the findings of Mr Amano's team.
The Fukushima plant's operator Tokyo Electric Power (Tepco) said it was not ruling out the option of entombing the plant in concrete to prevent a radiation leak; a similar method was used at Chernobyl.
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Fractured Fukushima
- Reactor 1: Fuel rods damaged after explosion last Saturday
- Reactor 2: Damage to the core, prompted by a blast on Tuesday, helped prompt raising of the nuclear alert level
- Reactor 3: Contains plutonium, core damaged by explosion on Monday; roof blown off building; water level in fuel pools said to be dangerously low
- Reactor 4: Hit by explosion on Tuesday, fire on Wednesday; roof blown off building; water level in fuel pools said to be dangerously low
- Reactors 5 & 6: Spent fuel pool temperatures way above normal levels
Military fire trucks sprayed the plant's overheating reactor units for a second day on Friday.
Water in at least two fuel pools - in reactor buildings 3 and 4 - is believed to be dangerously low, exposing the stored fuel rods.This increases the chance of radioactive substances being released from the rods.
An electricity line has been bulldozed through to the site and engineers are racing to connect it, but they are being hampered by radiation.
The plant's operators need the power cable to restart water pumps that pour cold water on the reactor units.
Military helicopters which dropped water from above on Thursday have been kept on standby.
Televised footage of the airdrops had shown much of the water blowing away in the wind.
http://www.bbc.co.uk/news/world-12783832
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