Japan has raised the severity level of a nuclear crisis at a quake-hit nuclear power plant, the UN nuclear watchdog has reported.
An entry on a monitoring website on Friday gave the accident at the Fukushima Daiichi site in the northeast of the country a level 5 rating, up from level 4 previously on a 1-7 scale.
The hallmarks of a Level 5 emergency are severe damage to a reactor core, release of large quantities of radiation with a high probability of "significant" public exposure or several deaths from radiation.
Al Jazeera's Andrew Thomas, reporting from Osaka, said that the raising of the severity level is "quite an admission by the Japanese government".
"They are very cautious, trying to say that this is anything more than a serious but contained accident, so this is a fairly big development."
The level was raised as teams of Japanese workers and troops battled to prevent a meltdown at the plant, 250km from Tokyo.
Chinook military helicopters have dumped tonnes of water on the plant in a desperate bid to cool reactors and prevent a catastrophic meltdown.
Fire engines were also put into action to douse fuel rods inside reactors and in containment pools to stop them from degrading due to exposure to the air and emitting dangerous radioactive material.
The fuel-rod pools contain used rods that have been withdrawn from reactors yet remain highly radioactive.
They are immersed in cooling water for many years until they shed enough heat to become manageable for storage.
Water in one of the pools was evaporating because of the rods' heat, and temperatures were slowly rising in two other pools because coolant pumps were knocked out by the March 11 quake and tsunami, experts said.
They warned that if the tanks run dry and leave the fuel rods exposed, the rods could melt or catch fire, creating potentially lethal levels of radiation.
Bid to restore power
At the same time, more than 300 engineers were focused on restoring the power supply to the stricken plant in an attempt to reactivate its cooling system.
The nuclear safety agency said early on Friday that plant operator Tokyo Electric Power Company (TEPCO) had managed to plug a cable from a regional power firm to the plant.
"Electricity can now be supplied," an agency spokesman said.
A further 1,480m (5,000ft) of cabling was being laid inside the complex on Friday evening, as officials said they hoped first to reconnect power to reactor number two, then to one, three and four during the weekend.
Even if TEPCO manages to connect the power, it is not clear the pumps will work as they may have been damaged in the earthquake or subsequent explosions.
Japanese engineers conceded on Friday that burying the nuclear plant in sand and concrete may be a last resort to prevent a catastrophic radiation release, the method used to seal huge leakages from Chernobyl in 1986.
The UN nuclear watchdog separately said the situation had not worsened "significantly" over the past 24 hours but warned it would be premature to talk about a ray of hope.
Flight of foreigners
Meanwhile, more foreign governments urged their citizens to steer clear of northeast and the capital Tokyo, Japan on Friday observed a minute's silence in memory of those who died.
Britain, France, Germany, Australia and New Zealand were among the nations advising their nationals to leave Tokyo and shun the northeast region.
The Japanese government has told people living up to 10 kilometres beyond a 20-kilometre exclusion zone around the crippled plant to stay indoors. More than 200,000 people have already been cleared from the zone.
Al Jazeera's Harry Fawcett reports from Noda on how tsunami preparations helped saved thousands of lives in the devastated town
The government has said radiation levels from the plant posed no immediate health threat outside the exclusion zone despite slightly elevated levels detected in Tokyo over the past few days.
US officials, however, warned citizens living within 80km of the plant to evacuate or seek shelter. The first US charter flight took off for Taiwan carrying almost 100 people, mostly families of US personnel.
The evacuation plans came against a background of mounting concern over the possibility of a nuclear catastrophe.
"The site is effectively out of control," Guenther Oettinger, the European Union's energy chief, told a European Parliament committee, a day after he said Japan was facing "apocalypse".
France's Nuclear Safety Authority said the disaster now equated to a six on the seven-point international scale for nuclear accidents, ranking the crisis second in gravity only to the level-seven Chernobyl disaster in 1986.
Steven Chu, the US energy secretary, said the events in Japan "actually appear to be more serious" than the 1979 accident at Three Mile Island, a partial reactor meltdown that led to small releases of radioactivity.
"To what extent we don't really know now," Chu said in Washington.
Worst case scenarios would involve millions of people in Japan threatened by exposure to radioactive material, but prevailing winds are likely to carry any contaminated smoke or steam away from the densely populated Tokyo area to dissipate over the Pacific Ocean.