Radiation has spread from damaged reactors at the Fukushima Daiichi nuclear plant following an explosion at one unit and a fire at another, Japanese government officials said early Tuesday.
A spokesman for the government said radiation levels at areas around the plant reached levels high enough to pose a health risk.
Several hours after the explosion and fire, elevated levels of radiation were detected in Tokyo, 175 miles away, though government officials said there was no health risk there.
The explosion at unit 2 and the fire at unit 4 of the plant, where units 1 and 3 also have exploded since the powerful earthquake and tsunami hit Japan on Friday, have Japanese officials "freaked out," a senior U.S. official said.
"The level seems very high, and there is still a very high risk of more radiation coming out," Prime Minsiter Naoto Kan said.
Kan said most people have left the 20-kilometer evacuation zone around the plant, and he advised people within a 30-kilometer (19-mile) radius to stay indoors to avoid possible radiation poisoning.
"It is likely that the level of radiation increased sharply due to a fire at Unit 4," Chief Cabinet Secretary Yukio Edano said. "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."
Still, the warning to residents within the 19-mile radius was dire.
"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," he said. "These are figures that potentially affect health. There is no mistake about that."
While the previous explosions at Fukushima Daiichi reactors Nos. 1 and 3 were hydrogen blasts caused by a buildup of steam in the reactor units, the new blast at reactor No. 2 has officials unsure of the cause.
In addition, the fuel rods in the reactor were melting, a senior U.S. official said, though the situation was not described as a meltdown.
Half of the fuel rods were exposed, not immersed in water, and the suppression pool, which holds the water used to keep the rods cool, seemed to be damaged, according to Tokyo Electric Co., which runs the plant, and government officials.
The U.S. official said water being pumped in is disappearing faster than it would if it only were caused by evaporation, which suggests there may be a leak in the reactor's containment vessel. But, the official said, it also could be that there is so much pressure inside the reactor that it is hard to pump in water.
The explosion, which occurred at 6:10 a.m., came shortly after the International Atomic Energy Agency announced that the reactors at the Fukushima Daiichi plant were shut down.
"There is no longer [a] chain reaction of nuclear material," said IAEA director general Yukiya Amano, according to The Associated Press. "Reactor vessels and primary containment vessels ... stay intact. The release of radioactivity is limited."
Officials also reported, according to the AP, that containment systems in the reactors appeared to be working and radiation levels around the plant were going down -- but that was before the explosion and fire.
The Japanese government formally has asked the U.S. Nuclear Regulatory Commission for help in stabilizing its troubled nuclear reactors in the wake of the country's massive earthquake and tsunami.
The NRC sent two boiling water reactor experts to Japan as part of a team of aid workers to help in the recovery efforts.
A number of nuclear reactors continue to deteriorate at the Fukushima Daiichi plant, raising worries of a nuclear meltdown.
Officials had grown increasingly worried about Fukushima Daiichi nuclear plant unit 2 after two hydrogen explosions in three days occurred at the plant, and the unit lost its ability to cool.
The fuel rods on unit 2 became fully exposed for the second time Monday, a dangerous development in the effort to stop the reactor from melting down.
Japanese officials said a closed steam vent caused a dip in the water levels, allowing the rods to be exposed, The Associated Press reported.
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The exposure of the fuel rods means that the temperature in the reactor is likely to rise, allowing steam to form. The steam could lead to the creation of hydrogen, which is what caused the explosions at reactors 1 and 3.
Knowing how long the fuel rods have been exposed is key to understanding if there is a real chance of a meltdown, said Dr. Peter Hosemann, a nuclear energy expert and professor at the University of California at Berkeley.
"Having too much of the fuel rods exposed for too long of a time can lead to the core melt. Again, if a core melt happens, the reactor pressure vessel and the containment are designed to contain it," Hosemann said.
Japanese officials acknowledged that the fuel rods appeared to be melting inside all three of the reactors at the Fukushima plant.
"Although we cannot directly check it, it's highly likely happening," Edano told the Associated Press.
Experts said that the melting of the fuel rods should not be seen as an indication of imminent danger.
"The melting of the fuel rods in and of itself is not an immediate threat to the life and health of the public…there's at least three layers [of protection]: 1. The fuel is inside cladding; 2. The fuel and cladding are inside the pressure vessel; and 3. The fuel and pressure vessel are inside a containment building and that containment building is holding up well and the pressure vessel is holding up well," University of California at Berkeley nuclear engineering professor Edward Morse said.
Officials first became concerned about unit 2 at the plant after pressure began rising in the reactor. Officials from the Tokyo Electric Power Co. told NHK News that the explosion at unit 3 might have damaged unit 2's cooling system.
Workers began pumping sea water into the reactor following the explosion Monday morning. The system pumping the sea water experienced a fuel loss, causing a dip in the water levels around the rods, NHK News reported. This led to the first exposure of the rods.
"They've had trouble with getting the pumps working, with site power in general," Morse said. "They've shipped in extra diesel generators and they may have to do some extra retrofit plumbing."
Workers had returned to pumping sea water when the fuel rods were exposed for a second time.
While unit 1, the first reactor to explode at the plant, appeared to be stable, unit 3, which exploded early Monday morning in Japan, reportedly had a leak in its bottom.
"We've never encountered this type of situation in history before," said Joe Cirincione, a nuclear policy expert. "We are beyond a reactor crisis at this point. This is a nuclear system crisis. The entire northern part of the Japanese nuclear power system has been delivered a body blow."
The leak at unit 3 made it difficult to keep the core of the reactor covered with sea water, Dr. Michio Kaku, a physicist, said.
"The situation is getting worse by the hour. We haven't hit bottom yet," Kaku said. "We now have reports that unit 3 suffered perhaps a 90 percent uncovering of the core -- this is unprecedented since Chernobyl."
Japanese officials insist that things are under control at the nuclear plant and that radiation levels are safe.
"They haven't stabilized the sea water yet," Kaku said. "Remember, they're hanging in there right there with the fingernails. This is how close we are to a full-scale meltdown. So it's stable in the sense that you're stable when you're hanging by your fingernails."
A trip to Japan's emergency command center revealed officials re-watching the explosion at the Fukushima plant. A gray plume of smoke could be seen rising from unit 3 on the command center's television.
The explosion of unit 3 injured 11 workers on Monday and could be felt as far as 25 miles away.
The Japanese government continues to test the nearly 180,000 people evacuated from around the Fukushima plant for exposure to radiation.
In the town of Koriyama, lines of families waited to be tested today.
Medical teams wearing white suits used Geiger counters and silver hand-held scanners to check everyone, aiming the scanners at little children holding their hands out.
One little girl clung to her Winnie the Pooh toy before being checked by the medical team.
The mother of the girl told ABC News that her family is OK but that surviving Japan's worst earthquake and massive tsunami has been like living through hell.
At least 160 people tested positive for exposure to radiation.
Radiation detected by U.S. helicopters flying 60 miles from the Japanese shore prompted the repositioning of the USS Ronald Reagan and seven other U.S. Navy ships, military officials said.
There is growing concern about whether radiation released in Japan could reach the United States. So far, winds blowing around the Fukushima plant are westerly or southwesterly winds, meaning any radiation that's released is being blown out to the Pacific Ocean.
The chairman of the U.S. Nuclear Regulatory Commission said officials see "a very low likelihood" and "a low probability" that there's any possibility of harmful radiation levels in the united States, Hawaii or any other U.S. territories.
"The lack of any harmful impacts to the U.S. is simply based on the nature of these reactors and the large distances, obviously, between those and any U.S. territories. So you just aren't going to have any radiological material that, by the time it traveled those large distances, could present any risk to the American public," NRC chairman Greg Jaczko said.
If a nuclear meltdown were to occur releasing significant amounts of radiation, it would take at least five to seven days to reach the west coast of the united States, according to a blog written by Jeff Masters, the director of meteorology for WeatherUnderground.com.
By the time the radiation would reach Hawaii or the western coast of the united States, much of the radiation capable of causing harm to people would have left the atmosphere or turned into precipitation and have been "rained out," Masters wrote in his blog.
Following the 1986 Chernobyl disaster, a cloud of radiation traveled over Europe. The radiation diffused as it traveled by wind and did not cause harm. Those closest to the Chernobyl plant were the ones who suffered the effects of the radiation.
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ABC News' Martha Raddatz, Luis Martinez, Lauren Pearle, Sunlen Miller and The Associated Press contributed to this report.