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.