Experts have moved to allay fears of a Chernobyl-style disaster after Japan ordered 140,000 people to stay indoors because of leaking radiation from a stricken nuclear power station.
A third explosion and a fire at the Fukushima Dai-ichi plant saw radiation spread along Japan's northeastern coast worsening the crisis caused by the 9.0-magnitude earthquake and tsunami which followed.
The disaster has so far killed 3,300 people, although the death toll is expected to top 10,000. Some 500,000 peopel are homeless and the world's third-largest economy has taken a battering.
Hundreds of aftershocks have shaken Japan's northeast and Tokyo since the original offshore quake, including one of magnitude 6.0 today. The quake could be felt in Tokyo where buildings swayed.
In a nationally televised statement this morning, Japanese prime minister Naoto Kan urged people within 30km of the Fukushima facility, 240km north of Tokyo, to remain indoors and conserve power. "I know that people are very worried but I would like to ask you to act calmly."
In addition to the three blasts that have occurred since Saturday, Japan’s chief cabinet secretary Yukio Edano said today a fire had broken out in a fourth reactor at the plant, and more radiation had been released. “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. “Please do not go outside. Please stay indoors. Please close windows and make your homes airtight.”
Japanese officials said the reactor fire was in a fuel storage pond - an area where used nuclear fuel is kept cool - and that "radioactivity is being released directly into the atmosphere."
Japan is considering asking for military help from the US to spray water from helicopters into the pool for the reactor, which had been shut down before the quake for maintenance. If the pool water boils and evaporates the fuel rods will be exposed.
The fuel rods are encased in safety containers meant to prevent them from resuming nuclear reactions, nuclear officials said. But they acknowledged that there could have been damage to the containers. They also confirmed that the walls of the storage pool building were damaged.
The International Atomic Energy Agency (IAEA) said radioactivity levels fell over a six-hour period today. It said about 150 people had received monitoring for radiation levels and that measures to "decontaminate" 23 of them had been taken.
The Vienna-based agency said a radiation dose level of 11.9 millisieverts (mSv) per hour was observed at the main gate of the Fukushima nuclear power plant early today. Six hours later, the rate had declined to 0.6 mSv, the agency said. The IAEA uses the unit to measure doses of radiation received by people. It quantifies the amount of radiation absorbed by human tissue. Exposure to over 100 mSv a year is a level that can lead to cancer, the World Nuclear Association says. People are exposed to natural radiation of 2-3 mSv a year.
The United Nations weather agency said winds were dispersing radioactive material over the ocean and that there is no danger for Japan or the region "for now".
Tokyo reported slightly elevated radiation levels, but officials said the increase was too small to threaten the 39 million people in and around the capital.
Radiation levels in the city of Maebashi, 100km north of Tokyo were up to 10 times normal levels, Kyodo news agency said. Only minute levels were found in the capital itself, which so far were "not a problem", city officials said.
The fear at the Fukushima Daiichi plant is of a major radiation leak after the quake and tsunami knocked out cooling systems. Authorities had previously been trying to prevent meltdowns in the complex's nuclear reactors by flooding the chambers with sea water to cool the reactors down.
Jiji news agency said the first explosion today damaged the roof and steam was rising from the complex.
The blasts could expose the population to longer-term exposure to radiation, which can raise the risk of thyroid and bone cancers and leukaemia. Children and foetuses are especially vulnerable.
There have been a total of four explosions at the plant since it was damaged in last Friday's massive quake and tsunami. The most recent were blasts at reactors Unit 2 and Unit 4.
Nuclear power plant operator Tohuku Electric Power Co (Tepco) has pulled 750 workers from its earthquake-hit Fukushima Daiichi plant since this morning. Kyodo news agency reported that Mr Kan was furious with Tepco executives for delays in informing his office about the latest explosion at the plant. It also said he asked the company not to withdraw employees from the facility.
Japanese media have became more critical of Mr Kan's handling of the disaster and criticised the government and Tepco for their failure to provide enough information on the incident.
Despite the government's pleas for calm, residents rushed to shops in Tokyo to stock up on supplies. Don Quixote, a multi-storey, 24-hour general store in Roppongi district, sold out of radios, torches, candles and sleeping bags.
In a sign of regional fears about the risk of radiation, Austria moved its embassy from Tokyo to Osaka. Air China and China Eastern Airlines cancelled flights to Tokyo and two cities in the disaster area. Germany’s Lufthansa is also diverting its two daily flights to Tokyo to other Japanese cities.
Several embassies advised staff and citizens to leave affected areas. Tourists cut short holidays and multinational companies either urged staff to leave or said they were considering plans to move outside Tokyo.
About 850,000 households in the north were still without electricity in near-freezing weather, and the government said at least 1.5 million households lack running water. Tens of thousands of people were missing.
"It's a scene from hell, absolutely nightmarish," said Patrick Fuller of the International Red Cross Federation from the northeastern coastal town of Otsuchi. "The situation here is just beyond belief, almost everything has been flattened. The government is saying that 9,500 people, more than half of the population, could have died and I do fear the worst."
Whole villages and towns have been wiped off the map by Friday's wall of water, triggering an international humanitarian effort of epic proportions.
The accident and the response have prompted criticism that authorities were ill-prepared and revived debate in many countries about the safety of atomic power. Chancellor Angela Merkel said all of Germany's nuclear plants which began operation before 1980 would be provisionally shut down.
Switzerland put on hold some approvals for nuclear power plants and Russian prime minister Vladimir Putin ordered a review of nuclear facilities and demanded faster development of the Sakhalin-3 oil and gas project to help meet energy demand from Japan.
The White House said President Barack Obama remained committed to nuclear energy.
Agencies