RADIATION levels near a quake-stricken nuclear plant are now harmful to human health, Japan's government says after explosions and a fire at the facility.

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

Tens of thousands have already been evacuated from a zone within a radius of 20km from the Fukushima No.1 plant, 250 kilometres northeast of Tokyo.

But Prime Minister Naoto Kan urged people living within 10km of the exclusion zone around the plant to stay indoors.

Early today a blast hit the number-two reactor there. And Mr Edano later said there was also an explosion which started a fire at the number-four reactor.

Although the number-four reactor was shut for maintenance when the quake and tsunami struck last Friday, "spent nuclear fuel in the reactor heated up, creating hydrogen and triggered a hydrogen explosion".

He said radioactive substances were leaked along with the hydrogen.

"Please keep in mind that what is burning is not nuclear fuel itself," Mr Edano said. "We'll do our best to put out or control the fire as soon as possible."

Similar hydrogen blasts had hit the number-one and number-three reactors on Saturday and yesterday. Buildings housing four of the six reactors at the plant, which opened in 1971, have now been hit by explosions.

Mr Edano said radioactive substances might spread outside the 20-30km area but would dissipate the farther they spread.

It was still unclear whether the container sealing the number-two reactor had been breached.

The plant operator initially told the nuclear safety agency that it had not been holed, but later said it was still checking for any breach.

Japan is frantically battling a nuclear emergency after Friday's massive earthquake and tsunami cut power to the 40-year-old plant and knocked out cooling systems.

Officials have struggled to prevent meltdowns at the damaged reactors, saying fuel rods may have been critically damaged by overheating.

But they have not reported the kind of radiation leakage that would accompany a major meltdown.

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

"There are very few people out in the streets", said Mako Sato, a cafe waitress in the town of Miharumachi just outside the evacuation zone. "They are either staying at home or in the evacuation centres.

"Since conditions surrounding the nuclear plant are so uncertain, I am worried. Food supplies are low and all that customers talk about is the quake and how scary it is, because we still feel aftershocks."

An employee at the Hotel Chisun in Koriyama said there were no visible signs of panic despite the nuclear crisis.

"Everyone is reacting calmly. But due to safety concerns after the quake we aren't accepting new business," the employee said. "

"There is very little food and convenience stores nearby are all closed. We are doing the best we can with our reserves."

Originally published as Japan reactor fire releases radiation