A third explosion and a fire at Japan’s Fukushima 1 nuclear reactor has released dangerous levels of radiation into the air above the plant.
The country’s prime minister Naoto Kan said radiation levels on the east coast had “risen considerably”.
People living less than 12 miles (20km) from the complex, which is 155 miles (250km) north of Tokyo, have been told to leave the area.
Tens of thousands of residents have already been evacuated from the zone.
A 30km no-fly zone has also been imposed around the power plant.
Chief cabinet secretary Yukio Edano told a news conference that the levels were high enough to damage people’s health.
About 150 people were monitored for their radiation levels and decontamination measures carried out on 23.
Those living within a 12 mile to 18 mile radius of the plant are being urged to stay indoors.
Mr Edano said: “Please do not go outside. Please stay indoors. Please close windows and make your homes airtight. Don’t turn on ventilators. Please hang your laundry indoors.”
American teacher, Mark Carney is one of those remaining indoors in his home in Fukushima City, about 60km from the nuclear plant.
He told Sky News he was concerned for his safety.
“I’m doing my best to stay inside, the information available is some what lacking. Five of my friends have chosen to leave because they were scared.
“It’s rather frustrating because we are not being told exactly what to do,” he said.
The Fukushima plant is in one of the provinces hit hardest by the 9.0 magnitude earthquake and subsequent tsunami on Friday.
After hydrogen explosions on Saturday and Monday, the problems have now spread to four out of the six reactors.
All non-essential workers have been evacuated from the damaged facility but around 50 others are attempting to cool the reactors down by injecting water.
The metropolitan government has said higher than normal radiation levels have been detected in Tokyo but not enough to harm human health.
The head of the International Atomic Energy Agency (IAEA) said the Japanese government has asked the agency for help and the US confirmed experts were being sent to the plants 150 miles north of Tokyo.
The IAEA said Japanese authorities had told them that radioactivity was being released “directly” into the atmosphere after the latest incident but that all units at the plant were in a “safe and stable condition.”
Winds were dispersing the radioactive material over the Pacific ocean, reducing the risks for Japan and other Asian countries, according to UN officials.
Sky News’ Anna Botting, in Sendai, said the authorities were warning people not to drink tap water and to take iodine to help prevent the threat of thyroid cancer.
The IAEA said around 185,000 people have been evacuated from 10 towns in the two Fukushima reactor regions.
It reported that 230,000 units of stable iodine has been distributed to evacuation centres near the Daiichi and Fukushima Daini power plants by officials.
The World Health Organisation said Japan was taking “all the necessary measures” to protect public health.
Meanwhile, the White House said it fully supported the Japanese approach to safety.
Four separate nuclear power stations – two in Fukushima and one each at Tokai and Onagawa – are operated by Tokyo Electric Power Company (Tepco) on the affected coast.
This is the worst nuclear crisis Japan has faced since the atomic bombing of Hiroshima and Nagasaki during World War II.
It is also the first time that such a grave nuclear threat has been raised in the world since Chernobyl.
The IAEA said that the incident rated level four – out of seven on its scale of seriousness. Chernobyl was a seven rating.
IAEA’s Japanese chief Yukiya Amano said “the possibility that the development of this accident into one like Chernobyl is very unlikely”.
France’s nuclear safety authority, however, said the situation was now at level six.
Malcolm Grimston, a nuclear expert from the Chatham House, told Sky News: “I think there is enormous concern amongst the nuclear community about what is happening in Japan.
“There are straws in the wind but until these plants get into a stable position of cooling and shutdown there are going to be more releases of activity.
“We are going to see some contamination of the local environment and for workers on site there are going to does that need us to keep an eye on their health.”
Air China has cancelled its flights from Beijing and Shanghai to Tokyo and Thailand has said it will randomly test food imports from Japan.
Russia said that radiation levels had risen slightly in the country’s Far East and the military was placed on alert to evacuate people from the Pacific Kuril archipelago and Sakhalin island if the situation worsened.
The nuclear incident in Japan has renewed concerns of atomic power, which has gained support in recent years as a future widespread replacement for fossil fuels energy.
As the nuclear crisis heads closer to meltdown, the death toll from the disaster has risen to 3,373, but officials have said at least 10,000 people may have died in Miyagi province alone.