- High levels of radiation found miles from nuke plant
- U.N. atomic chief calls for world to help Japan
- 6,500 dead, more than 10,300 missing following quake
- Officials working to fix power cable to stricken reactors
- Japan hails 'Samurai warriors' working at nuke plant
- Tiny amounts of radiation reach California
Experts said exposure for just six hours would result in absorption of the maximum level considered safe for a year, Japanese broadcaster NHK reported.
The country's science ministry said up to 0.17 millisieverts per hour have been detected 18 miles northwest of the Fukushima Dai-ichi plant, according to NHK.
Its officials tested radiation levels between from 9:20 a.m. and 3 p.m. local time on Thursday at 28 spots, in areas 12 to 37 miles from the plant, NHK said.
However, NHK reported that lower levels of radiation - 0.0183 to 0.0011 millisieverts per hour - were found at most observation points. These were higher than normal but posed no immediate threat to health.
A diplomat with access to U.N. radiation tracking said the fallout from the nuclear plant had reached Southern California , but emphasized it was at levels "about a billion times" beneath those harmful to health. He asked to remain anonymous as the data from the Comprehensive Test Ban Treaty Organization is not made public.
Japan reclassified the rating of the country's nuclear accident Friday from Level 4 to Level 5 on a seven-level international scale.
Nuclear experts have been saying for days that Japan had been underplaying the severity of the crisis.
The International Nuclear Event Scale defines a level 4 incident as having local consequences and a level 5 incident as having wider consequences. The level 5 puts the Fukushima accident on a par with the one at Three Mile Island in Pennsylvania in 1979.
France's nuclear safety authority earlier this week said the accident could be classified as a level 5 or 6 on the scale.
Video: 24 years later, radiation haunts Chernobyl (on this page)
Japan's Chief Cabinet Secretary Yukio Edano acknowledged Friday that the government was overwhelmed by the scale of last week's 9.0 magnitude earthquake and tsunami.
It set off the nuclear problems by knocking out power to cooling systems at the Fukushima plant. Since then, four of the troubled plant's six reactor units have seen fires, explosions or partial meltdowns.
"The unprecedented scale of the earthquake and tsunami that struck Japan, frankly speaking, were among many things that happened that had not been anticipated under our disaster management contingency plans," Edano said, admitting that information had not been shared quickly enough.
"In hindsight we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster," he said.
Edano said that Tokyo was asking the U.S. government for help and that the two were discussing the specifics.
"We are coordinating with the U.S. government as to what the U.S. can provide and what people really need," Edano said.
'Extremely serious accident'
Earlier, the U.N. atomic energy chief Yukiya Amano called Friday for the world to help Japan deal with the nuclear crisis.
"We see it as an extremely serious accident," Amano, the head of the U.N.'s International Atomic Energy Agency, told reporters Friday after arriving in Tokyo.
"This is not something that just Japan should deal with, and people of the entire world should cooperate with Japan and the people in the disaster areas," he said.
"I think they are racing against the clock," he said of the efforts to cool the complex.
Amano also urged Japanese Prime Minister Naoto Kan to give more detailed information on the crippled reactors.
At the stricken complex, military fire trucks began spraying the troubled reactor units again Friday morning, with tons of water arcing over the facility in desperate attempts to douse the units and prevent meltdowns that could spew dangerous levels of radiation.
Emergency teams at the stricken nuclear plant have been dubbed atomic "Samurai warriors" and "Kamikaze" - meaning "divine wind" - in Japan for their bravery in fighting the disaster, Euronews reported.
"The whole world, not just Japan, is depending on them," Tokyo office worker Norie Igarashi, 44, said.
A defense ministry official said that a U.S. military fire truck was standing by to help supply water to the crippled reactor units. Speaking on condition of anonymity because he was not authorized to speak to the media, the official said the vehicle would be driven by Japanese workers.
General Electric has sent also nuclear engineers to a Japanese emergency response center where they are working with Tokyo Electric Power Co to prevent a meltdown, a company spokesman told the Wall Street Journal. (Msnbc.com is a joint venture between NBC Universal and Microsoft. GE is a part owner of NBC Universal.)
GE designs nuclear reactors, including all six at the Fukushima Dai-ichi plant. GE said it has tapped a network of more than 1,000 current and retired engineers. The company has been directly advising the Japanese and U.S. governments, the paper said.
Both need water to keep their uranium cool and stop them from emitting radiation, but with radiation levels inside the complex already limiting where workers can go and how long they can remain, it's been difficult to get enough water inside.
Water in at least one fuel pool - in the complex's Unit 3 - is believed to be dangerously low, exposing the stored fuel rods. Without enough water, the rods may heat further and spew out radiation.
Four of the troubled Fukushima Dai-ichi plant's six reactor units have seen fires, explosions or partial meltdowns in the week since the tsunami.
Crucial to the effort to regain control over the Fukushima plant is laying a new power line to the plant, allowing operators to restore cooling systems.
The operator, Tokyo Electric Power Co., missed a deadline late Thursday but nuclear safety agency spokesman Minoru Ohgoda said Friday that workers hoped to complete the effort in 10 to 15 hours.
But the utility is not sure the cooling systems will still function. If they don't, electricity won't help.
Smoke was seen coming from the complex's Unit 2 Friday, but its cause was not known, the nuclear safety agency said.
An explosion had hit the building on Tuesday, possibly damaging a crucial cooling chamber that sits below the reactor core.
On Thursday, military helicopters dumped thousands of gallons of water from huge buckets onto the Unit 3 reactor.
Officials announced Friday they would not continue with the helicopter drops - televised footage appeared to show much of that water blowing away.
At the plant, the core team of 180 emergency workers has been rotating out of the complex to minimize radiation exposure.
More than 20 at the stricken plant have been injured, more than 20 have been exposed to radiation and two have gone missing during the battle to prevent a nuclear disaster, officials said Thursday.
U.S. Nuclear Regulatory Commission Chairman Gregory Jazko has warned that emergency staff at the stricken plant would face potentially "lethal doses" of radiation "in a very short space of time."
'Sense of dread'
Low levels of radiation have been detected well beyond Tokyo, which is about 140 miles south of the plant, but hazardous levels have been limited to the plant itself.
Still, the crisis has forced thousands to evacuate and drained Tokyo's normally vibrant streets of life, its residents either leaving town or holing up in their homes.
"I feel a sense of dread," said Yukiko Morioka, 63, who has seen business dry up at her lottery ticket booth in Tokyo. "I'm not an expert, so it's difficult to understand what's going on. That makes it scarier."
Police said more than 452,000 people made homeless by the quake and tsunami were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short.
Both victims and aid workers appealed for more help, as the chances of finding more survivors dwindled.
The State Department has warned U.S. citizens to consider leaving the country and offered voluntary evacuation to family members and dependents of U.S. personnel in the cities of Tokyo, Yokohama and Nagoya.
The Associated Press and Reuters contributed to this report.