Work to stabilize the damaged reactors at the Fukushima Daiichi nuclear power plant was temporarily halted early Wednesday because radiation leaking from the units made the situation unsafe, Japanese officials said.
Several hours after work was halted, officials were preparing to return to work but it was unclear whether workers were able to enter the plant, the Associated Press reported.
Radiation levels started to rise sharply after steam was seen escaping from unit 3 at the plant, which was damaged first by the 9.0 earthquake and tsunami that struck Japan Friday, and then by an explosion in the reactor.
There have been explosions in two other reactors at the plant, and two fires at a fourth unit, which was being used as a storage facility for radioactive material.
A Japanese government official also indicated for the first time that the containment vessels of all three of the reactors at the plant that exploded may be leaking, raising worries of dangerous radiation leaks.
The release of steam and work stoppage came after firefighters were able to put out a fire at unit 4 at the plant.
The International Atomic Energy Agency said Tuesday that radiation dose rates of up to 400 millisievert (mSv) per hour had been reported at the Fukushima plant site immediately following one of the explosions. A typical chest X-ray exposes an individual to about 0.02 mSv.
But after the steam was observed escaping from unit 3, radiation levels rose sharply, a government spokesman said.
Chief Cabinet Secretary Yukio Edano said there was a reading of 1,000 millisieverts, before the dose level began falling again to 600-800 millisieverts per hour, which is still considered unsafe.
"So the workers cannot carry out even minimal work at the plant now," Edano said. "Because of the radiation risk, we are on standby."
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Japan's Self-Defense Forces had attemped by helicopter to put water on unit 3 but radiation levels were unsafe to complete the mission according to NHK and the Kyodo news agency.
Also on Wednesday, in a rare address, Japan's Emperor Akihito expressed his concerns with the nuclear crisis and gave his condolences for the victims and told the country "not to give up" on national television, the Associated press reported.
Japanese Prime Minister Naoto Kan urged those living 12 to 19 miles around the plant (20-30 km) to stay indoors. The 140,000 people living within 12 miles of the plant have been evacuated. So far, 150 people from that area have tested positive for exposure to radiation.
"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," Chief Cabinet Secretary Yukio Edano said to the residents in the danger zone. "These are figures that potentially affect health. There is no mistake about that."
Heeding the government's word to evacuate, cars jammed highways and waited in lines four lanes wide to get gas. Aircraft were also affected by the damaged nuclear plant. The Japanese Transport Ministry imposed a 19 mile no-fly zone near the facility, allowing only aircraft involved in relief efforts to be exempt from the restriction.
"When this crisis started we compared it to what Americans are familiar with the 1979 Three Mile Island crisis and the 1986 Chernobyl crisis," said Joe Cirincione, a nuclear policy expert who is president of the Ploughshares Fund and an ABC News consultant. "This is way past Three Mile Island and we are heading into Chernobyl territory."
Just 50 of the plant's 800 workers remained at the plant Tuesday, fighting to keep the reactors cool by pumping sea water into them. Officials ordered most of the workers to leave the plant after the initially high levels of radiation were reported.
The explosion at Unit 2 has experts worried because the roof of Unit 2 did not suffer damage. In two previous hydrogen explosions at the plant, the roofs of those reactors were partially blown apart, giving an indication that the reactor's inner containment vessel remained intact and the radioactive fuel rods were still protected.
It's now believed the trapped pressure cracked Unit 2's containment vessel around the reactor's core, allowing radioactive material to seep out.
"What we're seeing is unprecedented in nuclear power history. It's made worse by way the Japanese build their reactors, they cluster them together. ... Many of the plants in Japan have four or more units, it's very efficient, so what it means is a disaster at one can avalanche into a complicated disaster next door," said Cirincione.
Following the explosion of unit 2, a fire broke out in the fuel pond of unit 4, a reactor that had not been of concern to officials. The fire was contained, but now radiation is believed to be leaking from that unit too.
"It's just as dangerous as the reactor core and they have the same kind of water level problems. If you can't cover them with water, those fuel rods will catch fire and they are even more radioactive," Cirincione said.
"If I had the ear of a Japanese prime minister," said Michiko Kaku, a theoretical physicist at the City University of New York, "I would suggest that he exercise the Chernobyl option, that is, put the Japanese Air Force on standby, assemble a fleet of helicopters, get sand, concrete and sandbag these reactors like what they did at Chernobyl.
"Sandbag the reactors ... entombing it in a sarcophagus of concrete that was the last resort that the Soviets used in 1986 and the Japanese may have to use that final option."
Hours after the explosion and fire at Daiichi, elevated levels of radiation were detected in Tokyo, 175 miles away, though government officials said there was no health risk there.
Naval personnel on the USS George Washington, a U.S. aircraft carrier assisting in the recovery efforts, detected low levels of radiation, prompting military personnel to take precautions that included limiting outdoor activity.
Radiation was detected Monday by U.S. helicopters flying 60 miles from the Japanese shore, prompting the repositioning of the USS Ronald Reagan and seven other U.S. Navy ships, military officials said.
On Monday, the Japanese government formally asked the U.S. Nuclear Regulatory Commission and the IAEA for help in stabilizing its troubled nuclear reactors in the wake of the country's massive 9.0 earthquake and tsunami.
Officials from the Nuclear Regulatory Commission said that Americans should not be concerned about the spread of radiation to the West Coast of the United States and Hawaii.
If a nuclear meltdown were to occur, releasing significant amounts of radiation, it would take at least six to 10 days to reach the West Coast.
By the time the radiation reached Hawaii or the West Coast, much of the radiation capable of causing harm to people would have left the atmosphere or turned into precipitation and have been "rained out," according to a blog written by Jeff Masters, the director of meteorology for WeatherUnderground.com.
Westerly winds around the plant continue blow east towards the Pacific Ocean.
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.
"Even after the Chernobyl accident, iodine did get into the milk in New York City, but it was very microscopic levels. We have not that much to fear in the United States," Kaku said.
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ABC News' Jim Hill, Juju Chang, Martha Raddatz, Luis Martinez, Lauren Pearle, Sunlen Miller and The Associated Press contributed to this report.