The latest fire to break out at a quake-hit Japanese nuclear power plant is under control, the government says.

"We have received information from TEPCO (the plant's operator) that the fire and smoke is now invisible and it appears to have gone out of its own accord," Minoru Ogoda, a spokesman for the state nuclear safety agency says.

The blaze broke out today on the fourth floor of the number-four reactor at the Fukushima No.1 plant, 250 kilometres northeast of Tokyo.

The Tokyo Electric Power Company confirmed at 5.45am Japan Time (9.45am NZT) today flames were seen coming out of the fourth floor of the building.

However, around 30 minutes later, the flames could no longer be seen, TEPCO says.

TEPCO had informed the fire service, however the building is reportedly inaccessible due to high radiation levels, between 300-400 millisieverts per hour.

No injuries have been reported.

Fire near spent fuel rod storage

The Disaster Taskforce headquarters is trying to work out the cause of the fire, the NHK news broadcaster reports, although it appears it may have been due to a hydrogen explosion.

The fire was on the fourth floor of the building where the spent fuel rod storage is located, the same area as a fire yesterday.

Unit 4 was shut down for a routine, planned maintenance outage in November of last year and all fuel was transferred to the spent fuel pool following the earthquake.

It is believed the fuel rods became exposed as water in the storage pool has decreased, causing a build up of hydrogen which may have led to the blaze.

On Monday Tepco reported the water in the pool was 84 degress Celcius, double the normal temperature.

Spike in radiation

Yesterday's fire at unit 4 at 9.30am Japan time (12.30pm NZT) led to a sharp rise of radiation levels at the plant - at 10.22am Japan time (2.22pm NZT) a radioactivity monitoring post near the unit 3 reactor showed 400 millisieverts per hour, 400 times the amount an ordinary person is exposed to in a year, the Japan Times reported.

The fire yesterday blazed for two hours, damaging a wall and a roof, and went out on its own.

70 per cent of nuclear fuel rods damaged

Meanwhile, Japan's nuclear safety agency says 70 per cent of the nuclear fuel rods in an earthquake-stricken reactor may have been damaged following an explosion.

An agency spokesman, Minoru Ohgoda, said today that the damage occurred in Unit 1 of the Fukushima Daiichi nuclear complex. The facility has suffered explosions and fires since Friday's massive quake and tsunami damaged its cooling system.

Ohgoda said that "it's likely that roughly about 70 per cent of the fuel rods may be damaged".

But he said "we don't know the nature of the damage, and it could be either melting, or there might be some holes in them."

'What the hell is going on?'

Earlier today a French nuclear official labelled the Japanese nuclear crisis at a level just below Chernobyl, as the Japanese Prime Minister appeared to be losing faith in his nuclear plant operators Tokyo Electric Power Company.

Radiation levels at the crippled Fukushima Daiichi nuclear plant spiked late yesterday before retreating slightly overnight.

Andre-Claude Lacoste, president of France's nuclear safety authority, refused to call the situation a "nuclear catastrophe" but said the situation was far more severe than the level 4 incident classification given by the International Atomic Energy Agency (IAEA).

"It's clear we are at Level 6, that's to say we're at a level in between what happened at Three Mile Island and Chernobyl," Lacoste said.

His remarks came as Japanese news agency Kyodo reported Prime Minister Naoto Kan was angry at not being told of a blast at Fukushima Daiichi's number 2 reactor yesterday.

"The TV reported an explosion, but nothing was said to the prime minister's office for more than an hour," Kan was reported to have said. "What the hell is going on?"

The director general of the International Atomic Energy Agency, Yukiya Amano, also said he wanted more timely and detailed information from Japan - his first hint at frustration with the pace of updates from authorities in his home country.

"The problem is very complicated, we do not have all the details of the information so what we can do is limited," Amano told a news conference overnight. "I am trying to further improve the communication."

Amano said he still believed the situation was different from that of the Chernobyl disaster, even though recent events "are worrying" and around 200,000 people had been evacuated from around the affected areas.

Panic sweeps through Tokyo

As the crisis escalated, panic swept Tokyo this morning after a rise in radiation levels.

Some residents left the capital, and others were stocking up on food and supplies.

Several embassies advised staff and citizens to leave affected areas, tourists cut short vacations and multinational companies urged staff to leave or said they were considering plans to move outside Tokyo.

Radios, torches, candles, fuel cans and sleeping bags were sold out in the Don Quixote, a multi-storey, 24-hour general store in downtown Tokyo.

Tourists had enough

Tourists such as Christy Niver, of the United States, said they had enough and were leaving.

Her 10-year-old daughter, Lucy, was more emphatic. "I'm scared. I'm so scared I would rather be in the eye of a tornado," she said. "I want to leave."

New Zealand's Ministry of Foreign Affairs and Trade is now recommending against all tourist and other non-essential travel to areas in Japan, including Tokyo, affected by the earthquake and tsunami. China said it would evacuate expat citizens from the worst-hit areas of Japan.

Contamination fears

Kyodo News said "minute levels" of radiation had been detected in Tokyo, and radiation levels in Saitama, near Tokyo, were 40 times normal levels - not enough to affect human health, but enough to fuel panic in the city of about 12 million people.

Fears of radioactive contamination from the crippled nuclear plant in tsunami-ravaged Japan escalated after a third reactor explosion and a fire in another.

The International Atomic Energy Agency said last night Japanese officials had told it the reactor fire was in the storage pond - a pool where used nuclear fuel is kept cool - and that "radioactivity is being released directly into the atmosphere".

In the worst case, the reactor's core would completely melt down, a disaster that could spew large amounts of radioactivity into the atmosphere.

Prime Minister Kan said dangerous levels of radiation had spread from four reactors of the Fukushima nuclear plant, one of the hardest-hit in Friday's 9.0-magnitude earthquake and tsunami that has killed at least 20,000 people.

Residents told to stay indoors

Mr Kan warned of the dangers of more leaks and told people living within 30km of the Daiichi complex to stay indoors to avoid radiation sickness.

About 70,000 people have been evacuated from a 20km radius and 140,000 remain in the zone for which the new warning was issued.

"The level seems very high and there is still a very high risk of more radiation coming out," Mr Kan said. "We are making utmost efforts to prevent further explosions and radiation leaks."

It is the worst nuclear crisis Japan has faced since the atomic bombing of Hiroshima and Nagasaki during World War II. And it is the first time such a grave nuclear threat has been raised since a nuclear power plant in Chernobyl, Ukraine, exploded in 1986.