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.
Panic-buying Further heavy snowfall overnight 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.
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.
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.
He said his team would monitor radiation in Tokyo and in the area around the Fukushima Daiichi site, which has been rocked by a series of explosions.
The IAEA announced it would hold a special board meeting on Monday to discuss Mr Amano's findings.
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.
Military fire trucks have been spraying the plant's overheating reactor units for a second day.
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.
