TOKYO, Japan (UPDATE) - Radiation near a quake-hit nuclear plant reached levels harmful to human health, Japan's government said after two explosions and a fire at the crippled facility Tuesday.
Four of the six reactors at the Fukushima No.1 plant, 250 kilometres (155 miles) northeast of Tokyo, have now overheated and sparked explosions since Friday's massive earthquake and tsunami knocked out cooling systems.
"There is no doubt that unlike in the past, the (radiation) figures are at 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 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 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."
Higher than normal radiation was detected in Tokyo on Tuesday, but a city official said it was not considered at a level harmful to human health and the level fell later in the day.
Edano said at a briefing later Tuesday that the radiation level at the plant's main gate had fallen sharply.
He said the earlier high reading might have been caused by contaminated debris from a explosion Monday rather than by a persistent leak.
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 which started a fire.
The plant operator Tokyo Electric Power Co (TEPCO) said the blaze was extinguished later in the morning with the help of US troops.
Edano said the remaining two reactors at plant, five and six, had begun overheating slightly and were being closely watched.
The number-four reactor was shut for maintenance when the quake and tsunami struck last Friday, but "spent nuclear fuel in the reactor heated up, creating hydrogen and triggered a hydrogen explosion", Edano said.
He said cooling seawater was being pumped into reactors one and three, which were stabilising. Reactor two was also getting cooling water but it was too early to say whether it was stable.
Radiation levels near the reactors as of mid-morning ranged from 30 to 400 millisieverts. A single dose of 1,000 millisieverts -- or one sievert -- causes temporary radiation sickness such as nausea and vomiting.
The government spokesman said radioactive substances might spread outside the 20-30 kilometre area around the plant but would dissipate the farther they spread.
The UN's nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance in the wake of the disaster caused by a quake now measured at a magnitude of 9.0.
But its chief Yukiya Amano said the crisis was unlikely to turn into a new Chernobyl.
The blasts 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.
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."