Emergency crews are still battling to cool overheated nuclear fuel at a stricken nuclear plant a week after the devastating earthquake and tsunami struck Japan.
Japan's nuclear safety agency raised the alert level for the Fukushima crisis, reclassifying the nuclear accident from Level 4 to Level 5 on a seven-level international scale.
A Level 4 incident is defined as having local consequences and a Level 5 as having wider consequences. The 1986 Chernobyl disaster was rated as 7.
Military fire trucks sprayed the reactor units for a second day, with tons of water arcing over the facility in desperate attempts to prevent the fuel from overheating and emitting dangerous levels of radiation.
"I think they are racing against the clock," Yukiya Amano, the head of the UN's International Atomic Energy Agency said of the efforts to cool the complex, after arriving in Tokyo.
Sirens wailed along the devastated coastline to mark exactly one week since the twin disasters.
The government admitted it was slow to respond to the disasters that the prime minister called a "great test for the Japanese people".
The admission came as Japan welcomed US help in stabilising its overheated, radiation-leaking nuclear complex.
Nuclear experts have been saying for days that Japan was underplaying the severity of the problems at the Fukushima Dai-ichi power plant.
Prime Minister Naoto Kan vowed that the disasters would not defeat his country.
"We will rebuild Japan from scratch," he said in a nationally televised address, comparing the work with the country's emergence as a global power from the wreckage of the Second World War.
"In our history, this small island nation has made miraculous economic growth thanks to the efforts of all Japanese citizens. That is how Japan was built," he said.
Last week's 9.0 quake and tsunami set off a cascade of problems by knocking out power to cooling systems at the nuclear plant.
Since then, four of Fukushima's six reactor units have seen fires, explosions or partial meltdowns.
The unfolding disaster has left more than 6,900 dead - exceeding the 1995 earthquake in Kobe, Japan, that killed more than 6,400.
Most officials, however, put estimates of the dead from last week's disasters at more than 10,000.
It has also led to power shortages, factory closures, hurt global manufacturing and triggered a plunge in Japanese stock prices.
Chief Cabinet Secretary Yukio Edano admitted that Japan was not prepared for what happened.
"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," he said.
"In hindsight, we could have moved a little quicker in assessing the situation and co-ordinating all that information and provided it faster," he said.
Emergency crews are facing two challenges: cooling the nuclear fuel in reactors where energy is generated and cooling the adjacent pools where thousands of used nuclear fuel rods are stored in water.
Both need water to stop their uranium from heating up and emitting radiation, but with radiation levels inside the complex already limiting where workers can go and how long they can stay, it has 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. Without enough water, the rods may heat further and spew radiation.
"Dealing with Unit 3 is our utmost priority," Edano told reporters.
Hidehiko Nishiyama of the nuclear safety agency said the rating was raised when officials realised that at least 3 % of the fuel in three of the complex's reactors had been severely damaged. That suggests those reactor cores have partially melted down and thrown radioactivity into the environment.
While Tokyo has welcomed international help for the natural disasters, the government initially balked at assistance with the nuclear crisis. That reluctance softened as the problems at Fukushima multiplied.
Today Edano said Tokyo was asking Washington for help and that the two were discussing the specifics of the problem.
The US said its technical experts are now exchanging information with officials from Tokyo Electric Power Co which owns the plant, and with government agencies.
A US military fire truck was also used to help spray water into Unit 3, according to air force Chief of Staff Shigeru Iwasaki, though the vehicle was apparently driven by Japanese workers.
The Tokyo Fire Department said five of their trucks have joined in dousing operations at the unit.
The US has also conducted overflights of the reactor site, strapping sophisticated pods onto aircraft to measure airborne radiation. Two tests gave readings that US Deputy Energy Secretary Daniel Poneman said reinforced the US recommendation that people stay 50 miles (80 kilometres) away from the Fukushima plant.
Low levels of radiation have been detected well beyond Tokyo, which is 140 miles (220 kms) south of the plant, but hazardous levels have been limited to the plant itself.
The crisis has forced thousands to evacuate and drained Tokyo's normally vibrant streets of life, its residents either leaving town or staying in their homes.
The Japanese government has been slow in releasing information on the crisis. In a country where the nuclear industry has a long history of hiding safety problems, this has left many people, in Japan and among governments overseas, confused and anxious.
In the disaster zone, tsunami survivors, rescue workers and ordinary people observed a minute of silence today at 2:46 p.m. - the moment a week ago when the quake struck. Many were bundled up against the cold. As a siren blared, they lowered their heads and clasped their hands in prayer.
In the largely destroyed town of Hirota, 70-year-old Tetsuko Ito wept as she hugged an old friend she met at a refugee centre. One of her sons was missing and another had been evacuated from his home near the Fukushima complex.
"Every day is terrifying. Is there going to be an explosion at the reactor? Is there going to be word my other son is dead?" she said.
She searched for her missing son for three days, then her car ran out of petrol.
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.
About 343,000 Japanese households still do not have electricity and about one million have no water.
At times, Japan and the US have offered starkly differing assessments over the dangers at Fukushima.
US Nuclear Regulatory Commission Chairman Gregory Jazcko said it could take days and "possibly weeks" to get the complex under control. He defended the US decision to recommend a 50-mile (80-kms) evacuation zone for its citizens, wider than the 12-mile (20-kms) band Japan has ordered.
Crucial to the effort to regain control over the Fukushima plant is laying a new power line to the complex, allowing operators to restore cooling systems. Tokyo Electric missed a deadline late on Thursday, said nuclear safety agency spokesman Minoru Ohgoda.
Power company official Teruaki Kobayashi warned that experts will have to check for anything volatile to avoid an explosion when the electricity is turned on.
"There may be sparks, so I can't deny the risk," he said.
Even once the power is reconnected, it is not clear if the cooling systems will still work.
The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.Reuse content