Radiation leaks from the nuclear power plant damaged by the Japan earthquake pose a health risk, officials warn.
Four hydrogen explosions have occurred at the Fukushima Dai-ichi plant following cooling system failures caused by the 9.0 magnitude earthquake and tsunami last Friday.
After a fire broke out in a fourth reactor at the plant and more radiation released, Prime Minister Naoto Kan urged those within 19 miles of the area to stay indoors.
Mr Kan said: "The level seems very high, and there is still a very high risk of more radiation coming out."
The official death toll from the disaster has topped 2,400, with tens of thousands of people missing.
Some 800 non-essential staff have been evacuated from the nuclear plant, while 50 workers remained at the complex to try to cool the reactors with water.
Chief cabinet secretary Yukio Edano has warned people to stay inside and keep their homes airtight.
"Now we are talking about levels that can damage human health. These are readings taken near the area where we believe the releases are happening. Far away, the levels should be lower,” he said.
Experts have warned that radiation material could contaminate food and water resources, with children and unborn babies most at risk of possibly developing cancer.
They said exposure to radioactive materials has the potential to cause various kinds of cancers including thyroid cancer, bone cancer and leukaemia.
However, they added more accurate measurements of the level of radioactivity in the region are needed before a proper risk assessment can be given.
Lam Ching-wan, chemical pathologist at the University of Hong Kong, said: "The explosions could expose the population to longer-term radiation, which can raise the risk of cancer.
“For some individuals even a small amount of radiation can raise the risk of cancer. The higher the radiation, the higher the risk of cancer.”
However, she added: "Very acute radiation, like that which happened in Chernobyl and to the Japanese workers at the nuclear power station, is unlikely for the population.”
The latest explosion in one of the reactors suggests that one of the containment vessels may be "slightly ruptured", releasing more radioactivity into the atmosphere.
Mr Kan's official spokesman Noriyuki Shikata said workers at the plant were deliberately releasing gases containing low levels of radioactive material in order to relieve pressure within the reactors, but were confident this did not pose a risk to human health.
“There is water being injected and we are trying to control the situation,” he said.
The World Health Organisation said Japan was taking the correct measures to protect its population from radioactivity, including evacuations and stocking up on potassium iodide, an antidote to radiation.
Radiation is dangerous because it can cause changes or mutations in DNA, which may then go on to cause cancer.
While the human body can repair DNA changes or damage, a person is only safe if the repair process happens faster than the time it takes for the damaged or mutated DNA material to replicate.
The World Meteorological Organisation added that winds are now dispersing radioactive material from the Japanese nuclear crisis over the Pacific Ocean, away from Japan and other Asian countries.
Although effects have been offshore so far, the United Nations agency warned that weather conditions could change and it was closely monitoring satellite and other data.
"At this point, all the meteorological conditions are offshore so there are no implications, for Japan or other countries near Japan," Maryam Golnaraghi, chief of WMO's disaster risk reduction division, said.