After days of silence on the nuclear crisis in Japan, President Obama today downplayed concerns of a potential radiological impact in the U.S. and defended efforts to evacuate Americans from a zone around the crippled reactors four times larger than one imposed by Japan.
"I want to be very clear: We do not expect harmful levels of radiation to reach the U.S., whether it's the west coast, Hawaii, Alaska, or the U.S. territories in the Pacific," Obama said in the Rose Garden.
The president appeared to directly address reports that some Americans have begun stockpiling potassium iodide tablets, which are taken in emergencies to block the body's absorption of radioactive substances.
"The Centers for Disease Control and Prevention and public health experts do not recommend people in the U.S. take precautionary measures beyond staying informed," Obama said. "Going forward we will continue to keep the American people fully updated because I believe you must know what I know as president."
But Obama said the U.S. Nuclear Regulatory Commission and other scientific experts believe the danger from radiation spewing out of the Fukushima power plant extends well beyond the 12-mile radius Japanese officials have suggested, and he warned Americans living in Japan to evacuate within a 50-mile radius.
"Even as Japanese responders continue to do heroic work, we know that the damage to nuclear reactors at Fukushima Daiichi plant poses a substantial risk to people who are nearby," Obama said.
"That is why yesterday we called for an evacuation of American citizens who are within 50 miles of plant. That decision was based on a careful, scientific evaluation and the guidelines that we use to keep our citizens safe here in the U.S.," he said.
Meanwhile, the State Department authorized the voluntary departure of family members of U.S. government personnel late Wednesday and issued a warning to other Americans to "consider departing" Japan.
The first U.S. government-chartered plane ferrying nearly 100 family members of American diplomats and a few private citizens left Japan today for Taipei, Taiwan, with more flights planned for tomorrow depending on demand.
"These measures are temporary and dependents will return when the situation is resolved," Pentagon spokesman Col. Dave Lapan said.
For now, radioactivity levels are only extreme in the immediate vicinity of the power plant. And forecasts show that winds for the next three to five days would send any contamination away from population zones and out over the sea.
But one U.S. official told ABC News that the situation remains "horrifically worrisome," despite an apparent information gap between what Japanese power company officials are telling the Japanese government of the situation, and what U.S. has determined through independent reconnaissance.
Obama urged Japanese Prime Minister Naoto Kan during a phone call late Wednesday to take decisive steps to end the crisis because time is of the essencce.
"We're sharing with them expertise, equipment and technology so that courageous responders on the scene have the benefit of American teamwork and support," Obama said of the U.S. effort to assist Japanese engineers at the nuclear site.
Earlier today Obama visited the Japanese Embassy in Washington to sign a condolence book and reiterate the nation's commitment to the Japanese people during the crisis.
"My heart goes out to the people of Japan during this enormous tragedy. Please know that America will always stand by one of its greatest allies during their time of need," Obama wrote in the book.
"Because of the strength and wisdom of its people, we know that Japan will recover and indeed will emerge stronger than ever. And as it recovers, the memory of those who have been lost will remain in our hearts, and will serve only to strengthen the friendship between our two countries. May God Bless the people of Japan."
Japanese Ambassador Ichuro Fujisaki thanked Obama and the American people, saying "We are working 24 hours, and his assistance means a lot to all Japanese."
ABC News' Sunlen Miller, Luis Martinez and Akiko Fujita contributed to this report.