Fears of a catastrophe at the Fukushima nuclear power plant in Japan escalated following a third explosion and a fire in another reactor that caused radiation to rise to harmful levels.

Fifty to 70 technicians were left to struggle with a possible breach of the containment vessel in reactor No 2, where meltdown is feared, while staff were also battling earlier today to deal with a separate blaze which briefly flared at reactor No 4. All but essential staff were ordered away from the plant.

The blast on Tuesday is thought to be the most serious yet because it may have damaged the crucial containment vessel that surrounds the nuclear core and prevents radiation from leaking out. Any damage to it caused by the explosion increases the risk that radiation will be released from the core, though IAEA officials said the damage was minor and there appeared to be no immediate danger of radiation escaping.

The explosion was followed by a fire at a storage pond at reactor 4, where spent fuel rods were being cooled. That fire was extinguished, only to break out again early this morning as officials continued to worry that the pool could boil dry and expose fuel rods to the atmosphere.

Tokyo Electric Power, which operates the plant, said it was considering dispersing boric acid, a fire retardant, over reactor 4 from a helicopter.

Despite calls by the Japanese government for calm, panic spread as several airlines cancelled services to Tokyo, and flights out of Japan quickly sold out. Foreigners were fleeing the country while many local residents left the capital for the western city of Osaka.

Those among Tokyo's 12 million people who stayed remained indoors or stripped shop shelves of bread, bottled water, batteries and candles. Partly prompted by the prospect of power cuts, the hoarding threatened to hamper efforts to divert supplies to the area stricken by Friday's earthquake and tsunami, where millions are suffering food and water shortages.

The earlier fire at reactor No 2 led to a brief spike in radiation that reached a dose known to be dangerous to health, although levels dropped substantially later in the day. While the disaster has become the biggest nuclear accident since Chernobyl in 1986 and more serious than at Three Mile Island in 1979, experts stressed that even the worst-case scenario would still be on a fraction of the scale of Chernobyl in the Ukraine.

Japan's prime minister, Naoto Kan, urged the public to remain calm in a televised address, but ordered anyone still within the 13-mile exclusion zone to leave immediately, and the 140,000 residents within 19 miles to stay indoors.

Officials said that health risks to anyone further than 13 miles from the plant were minimal, but public confidence was diminishing. Survivors within the disaster zone expressed distrust in officials and the power station's operators.

The prime minister, already embattled before the disaster, is meanwhile under pressure over his handling of the crisis, particularly over the way the nuclear situation has spiralled despite reassurances from officials.The number confirmed dead or missing rose above 10,000, Japan's largest death toll in a natural disaster since the Great Kanto earthquake of 1923, Kyodo news agency said.

Forecasters warned of temperatures dropping to midwinter levels across the disaster zone, with snow predicted. Half a million people have been displaced by the disaster and evacuation from the nuclear zone, with Save the Children saying 100,000 children are affected. Shelters and hospitals are running low on fuel and blankets.

In Ishinomaki, Patrick Fuller of the International Federation of Red Cross said: "The tsunami engulfed half the town and many lie shivering uncontrollably under blankets. They are suffering from hypothermia having been stranded in their homes without water or electricity."China said it was organising a mass evacuation of its citizens from the north-east of Japan.

Tokyo was shaken by a 6.2 magnitude earthquake night, but it was not immediately clear if it was an aftershock. It was centred in Shizuoka prefecture, hundreds of miles south-west of the capital and inland. It was not immediately clear if it was related to last Friday's tremor.