An employee confirmed smoke was pouring from the roof of the number-four reactor at the plant, a TEPCO spokesman told reporters.

"We immediately informed local authorities and the fire department about it," the spokesman said. "We are battling the fire now."

A blaze and explosion hit the same reactor on Tuesday, causing a crack in the roof.

The government has separately reported apparent damage to part of the container shielding the number-two reactor at Fukushima 250 kilometres (155 miles) northeast of Tokyo.

Japan is frantically working to avert a nuclear meltdown after Friday's massive earthquake and tsunami cut power to the ageing plant and knocked out cooling systems.

On Saturday an explosion blew apart the building surrounding the plant's number-one reactor but the seal around the reactor itself remained intact, officials said.

On Monday, shortly after Prime Minister Naoto Kan said the plant was still in an "alarming" state, a blast at its number-three reactor shook the facility, injuring 11 people and sending plumes of smoke billowing into the sky.

Late Monday TEPCO said fuel rods at the number-two reactor were almost fully exposed after a cooling pump there temporarily failed.

earlier related report
Japan races to contain nuclear crisis
Tokyo (AFP) March 16, 2011 - Japanese crews battled Wednesday to avert a nuclear disaster and said they may pour water from helicopters to stop fuel rods from being exposed to the air and releasing even more radioactivity.

Radiation near the quake-hit Fukushima No.1 plant has reached levels harmful to health and was high overnight, officials said, advising thousands of people to stay indoors after two explosions and a fire at the facility Tuesday.

Four of the six reactors at the crippled facility, 250 kilometres (155 miles) northeast of Tokyo, have now overheated and sparked explosions since Friday's massive earthquake and tsunami knocked out their cooling systems.

The blasts at the seaside plant have shattered buildings housing the reactors but have apparently not penetrated the steel and concrete containers surrounding the fuel rods, reducing the risk of massive contamination.

Workers have used fire-fighting equipment to pump seawater into the reactors -- and fears have spiked sharply after separate containment pools holding spent fuel rods at reactor number four started to heat up, threatening to run dry.

If the water in the deep pools evaporates, this would expose the fuel rods to the air, destroying them and sending radioactive materials into the air.

"We have no options other than to pour water from a helicopter, or to spray water from the ground," a spokesman for operator Tokyo Electric Power Co (TEPCO) said on television.

"We have to take action tomorrow or the day after."

The water in the containment pool of reactor number four may have been boiling earlier, Kyodo News reported.

Worsening levels of radiation on Tuesday forced the company to pull out most of its hundreds of workers who have been battling the emergency.

They later evacuated the plant's central control room and were now monitoring the site remotely, Kyodo reported.

The head of the UN nuclear watchdog said there might be limited core damage at the second reactor but repeated that he did not think it would turn into a Chernobyl-scale disaster.

"I continue to think that the Chernobyl and Fukushima reactors are different," Yukiya Amano said, reiterating that unlike Chernobyl, Fukushima had a primary containment vessel, and the reactor had shut down automatically when the earthquake hit, so there was no chain reaction going on.

Amano said the International Atomic Energy Agency was still discussing with Japan what sort of experts to send to Fukushima.

Tens of thousands have already been evacuated from within a radius of 20 kilometres (12 miles) of the 40-year-old plant, and Prime Minister Naoto Kan urged people living within 10 kilometres of that zone to stay indoors.

"There is no doubt that unlike in the past, the (radiation) figures are at the level at which human health can be affected," chief government spokesman Yukio Edano said Tuesday.

There was a danger of further leakage, Kan said.

"Please stay indoors, close windows and make your homes airtight," Edano urged residents during a press briefing. "Don't turn on ventilators. Please hang your laundry indoors."

Explosions hit the buildings housing reactors one and three Saturday and Monday. On Tuesday, a blast hit reactor two at the crippled plant and there was also an explosion at reactor four that started a fire.

TEPCO said the blaze was extinguished later in the morning with US help. The Pentagon said it provided two fire trucks for the Japanese to use.

Radiation levels were monitored at 4.548 millisieverts per hour at 11:00 pm (1400 GMT) and rose further to 7.966 millisieverts less than an hour later at the plant's entrance, Jiji Press reported, quoting TEPCO officials.

Levels fluctuated throughout the day.

A single dose of 1,000 millisieverts -- or one sievert -- causes temporary radiation sickness such as nausea and vomiting.

A US naval flotilla helping with quake relief efforts stepped up precautions against radiation, moving its ships and aircraft to avoid the winds from Fukushima and monitoring aircrews for contamination.

The continuing nuclear crisis has unnerved regional residents already struggling with the aftermath of the quake and tsunami.

Higher than normal radiation was detected in Tokyo on Tuesday, prompting many people to flee, but a city official said it was not considered at a level harmful to human health and the level fell later in the day.

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