Japan's top government spokesman says a surge in radiation at the crippled Fukushima nuclear power plant on Wednesday has made it too dangerous for workers to stay at the facility.

Chief Cabinet Secretary Yukio Edano said efforts to douse the crippled reactors with water had to be stopped and the workers withdrawn after radiation levels surged to dangerous levels.

"So the workers cannot carry out even minimal work at the plant now," Edano said. "Because of the radiation risk, we are on standby."

Edano said the Japanese government would likely ask the U.S. military for help, but wasn't specific about what kind of help.

Earlier on Wednesday, another fire broke out at the plant's No. 4 reactor, operator Tokyo Electric Power Co. said.

TEPCO said the new fire was essentially the same blaze that erupted Tuesday, and that it had not been fully extinguished.

Officials with Japan's nuclear safety agency said it was this fire that is believed to be the cause of the radiation surge.

TEPCO spokesman Hajimi Motujuku said the blaze erupted in the outer housing of reactor No. 4's containment vessel. Japan's nuclear safety agency also confirmed the fire, whose cause was not immediately known.

About three hours after the fire erupted, fire and smoke could no longer be seen at that reactor, but it wasn't clear if it had been put out by workers who were trying to contain it.

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The nuclear safety agency said TEPCO had been considering several measures such as spraying water and boric acid by helicopters and fire trucks into No. 4 to prevent spent fuel rods from reaching criticality again before work there was suspended.

The Wednesday fire comes a day after the plant was hit by a third explosion at one reactor and the fire at the fuel storage pool atop No. 4.

The agency also said Wednesday that about 70 per cent of the nuclear fuel rods at another of the Daiichi reactors may have been damaged.

Spokesman Minoru Ohgoda said the damage occurred at reactor No. 1. 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."

On Tuesday, more than 200,000 people living near a quake-damaged nuclear plant in northeastern Japan were urged to leave the area or stay indoors after government officials raised concerns about radiation levels around the facility and workers dealt with more problems there.

Prime Minister Naoto Kan urged people to stay calm but said there was a "high risk" that more radioactive material will be released as workers struggle to prevent further explosions and leaks at the plant.

He reiterated the call for people living within 20 kilometres of the plant to leave the area, and advised people living within 30 kilometres of the plant to seal themselves indoors.

Japanese officials also introduced a 30-kilometre no-fly zone around the nuclear plant to prevent planes from spreading the radiation.

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Seventy workers wearing protective gear have been at the complex, struggling with its myriad problems. They were being rotated in and out of the danger zone quickly to limit their radiation exposure.

Another 800 staff had earlier been evacuated from the site. The fires and explosions at the reactors have injured 15 workers and military personnel and exposed up to 190 people to elevated radiation.

The International Atomic Energy Agency said Tuesday it "remains concerned" over the situation at the plant, and is "seeking clarification on the nature and consequences" of the latest fire from Japanese officials.

Experts noted that much of the leaking radiation was apparently in steam from boiling water. It had not been emitted directly by fuel rods, which would be far more dangerous, they said.

A combination of handout satellite images shows the Fukushima Daiichi nuclear plant on Nov. 21, 2004, and on Monday. (Reuters )

After the Tuesday fire a Japanese official said the pool might still be boiling, though the reported levels of radiation had dropped dramatically by the end of the day.

Fourteen pumps have been brought in to get seawater into the reactors. They are not yet pumping water into reactor No. 4 but are trying to figure out how to do that.

Temperatures in at least two of the plant reactors, No. 5 and No. 6, were also slightly elevated.

"It's not good, but I don't think it's a disaster," said Steve Crossley, an Australia-based radiation physicist.

Even the highest detected rates were not automatically harmful for brief periods, he said.

Japanese officials had previously said radiation levels at the plant were within safe limits, and international scientists said that while there were serious dangers, there was little risk of a catastrophe like Chornobyl in Ukraine.