Shortly after, the U.S. State Department authorized the first evacuations of Americans out of Japan. Taking no chances, U.S. officials have ordered all Americans within 50 miles of the plant to evacuate.
It also issued a warning to Americans to avoid traveling to the quake and tsunami-ravaged nation.
Chartered planes will begin assisting American citizens wishing to leave the country.
"For a comparable situation in the United States we would recommend an evacuation to a much larger radius than has been provided in Japan," he said. "As a result of this recommendation, the ambassador in Japan has issued a statement to American citizens that we believe it is appropriate to evacuate to a larger distance, up to approximately 50 miles."
Japan has not expanded its radius of evacuation, which requires those living within 12 miles to leave their homes and those between 12 and 19 miles from the plant to stay indoors.
One hundred and eighty workers rotate shifts, working at the plant in teams of 50 men. The men have been nicknamed the "Fukushima Fifty."
One U.S. official told ABC News that they are urging the Japanese to get more people to help the workers inside the plant.
"This is very, very radioactive material...if there is core on the floor and containment penetration, there will be significant public health consequences," Ken Bergeron, a physicist and nuclear reactor safety expert, said.
Radiation levels were as high as 10 millisieverts per hour yesterday, the equivalent of getting a CT scan for every hour of exposure. Radiation levels have since dropped and the plant workers are planning to return to work, officials said.
The Japanese government has actually amended its national safety standard on how much radiation workers can be exposed to so that workers can return to the plant. The limit is now 250 millisieverts, 2.5 times the previous limit.
Emperor Akihito, a figure deeply respected in Japan, spoke for the first time yesterday since the March 11 earthquake that has left at least 5,300 people dead. He tried to ease worries about the country's nuclear crisis.
"With the help of those involved I hope things will not get worse," Akihito, 77, said.
He offered his condolences to a grieving nation where at least 9,083 people are still missing and 434,000 are homeless.
"It is important that each of us shares the difficult days that lie ahead," Akihito said. "I pray that we will all take care of each other and overcome this tragedy."
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ABC News' Martha Raddatz, David Muir, Jim Hill, Juju Chang, Dan Arnall and The Associated Press contributed to this report.