Two more reactors at the troubled Fukushima nuclear power plant in Japan have lost cooling functions as the plant's operators struggle to contain what threatens to become a meltdown.

Radiation levels rose to harmful levels at the facility earlier today following two reactor blasts and a fire at a spent fuel storage pond.

But government spokesman Yukio Edano says radiation levels have since fallen at the plant, 250 kilometres north-east of the capital Tokyo.

"This high level of radiation is just temporarily reading so in the long run it is not particularly dangerous and the reading is now getting lower," he said.

"We don't need to worry about this reading at the moment."

Mr Edano says the earlier high reading could have been caused by rubble from Monday's explosion at one reactor rather than from any continuing leak.

But he says cooling of the No. 5 and No. 6 reactors now appears not to be working properly, with slight rises in temperature recorded.

Explosions at the four other reactors at the plant followed the losses of their cooling systems after Friday's massive earthquake and tsunami.

A TEPCO official has told Kyodo that water in the pool storing spent nuclear fuel rods at the No. 4 reactor that caught fire this morning may be boiling, causing the water level to drop.

Mr Edano says today's explosion at the No. 4 reactor was caused by a build-up of hydrogen.

"Spent nuclear fuel in the reactor heated up, creating hydrogen and triggered a hydrogen explosion," he said.

The International Atomic Energy Agency (IAEA) says the fire at the reactor's spent-fuel pond has been extinguished.

The director of Japan's nuclear and industrial safety agency, Toshiro Bannai, warns reactors No. 5 and No. 6 are at risk of the same problems with their spent-fuel pools.

He says the temperature in the No. 4 pool more than doubled from its regular temperature of about 40 degrees Celsius.

"The alloy of the spent fuel, made from Zirconium alloy, these metals react with water when its temperature rises up to [about] 100 degrees [Celsius] to produce hydrogen," he said.

"The pools temperature is maybe 30-40 degrees [normally], it's not a high temperature ... but we need some cooling systems because spent fuel has a power of heat."

The government reported apparent damage to part of the container shielding the No. 2 reactor following a blast there earlier today. However, it was unclear whether the damage was caused by the blast.

Similar hydrogen blasts hit the No. 1 and No. 3 reactors on Saturday and Monday.

Radiation levels reached as high as 400 milisieverts an hour at the plant earlier today, thousands of times higher than readings taken before the latest blasts.

The level of radiation in Tokyo also rose above normal levels in the morning due to radiation emitted from the nuclear power plant but has now also fallen.

"We monitored a higher than normal amount of radiation in the morning in Tokyo but we don't consider it to be at a level where the human body is affected," said Sairi Koga, an official of the Tokyo Metropolitan Government.

He says the level fell from an average 0.809 micro-sieverts between 10:00am and 11:00am (local time) to 0.075 micro-sieverts four hours later.

Winds are now dispersing the radioactive material over the Pacific Ocean, away from Japan and other Asian countries, the World Meteorological Organisation (WMO) has said.

But the United Nations agency warned that although effects have been offshore so far, weather conditions could change and it was closely monitoring satellite and other data.

Trouble at the nuclear plant has sparked a rout on the stock market and panic buying in supermarkets.

Japan is struggling to cope with the enormity of the damage from Friday's record quake and the tsunami that raced across vast tracts of its north-east, destroying all before it.

The official death rose to 2,414, according to police, but officials warn at least 10,000 people are likely to have perished.

-AFP/Reuters