"Since the continued or increased release of wind-blown radioactive material cannot be ruled out, American citizens in Japan are advised to take prudent precautions against potentially dangerous exposure," Under Secretary of State for Management Patrick Kennedy told reporters.
The U.S. Department of Energy has deployed sensors over Japan to gather data about radiation levels on the ground.
"We are watching the situation at the plants continuously. We're trying to get some ground data on what the actual condition is," Deputy Secretary of Energy Dan Poneman said.
Jennifer Takahashi, a Canadian who lives in Sugito-machi, Japan, one hour north of Tokyo, was packing in a hurry Wednesday.
"I dont have a lot of time," she wrote to ABC News in an email, and noted that a Japanese neighborhood had come over to help watch her three children, ages 1, 2 an 7, as she rushed to get ready to leave.
"I am leaving in order to err on the side of caution as my children, particularly the youngest two, are so small and I worry about the risk of radiation to their health," she wrote. "I understand the current level of radiation in and around Tokyo is still well within a level that is safe but I worry about a potential rise of that level in the days to come."
Takahashi was not alone. "Many many non-Japanese members of an English playgroup I belong to based in Saitama are leaving or have left. If anything I am a bit on the late side of the so-called 'exodus' of foreigners from Japan," she said.
Takahashi said her neighborhood is considered to be safe from radiation, but said, "I am also unnerved by the credibility gap that appears to exist between what we are hearing from the Japanese government and media and the statements made in the international press."
She was scheduled to fly out of Narita today for Canada where she intends to stay for a month. Her husband, who is Japanese, stayed behind to keep working and help his family, Takahashi said.
Michael Miller, who is from Holmdel, N.J., retreated from Tokyo to Osaka, but is ready to leave the country if things get worse.
"For the next week, I will work via the internet and email, and monitor the situation from a safer distance. If things get really bad, I can always fly out from Osaka airport," he told ABC News.
Miller left Tokyo, where he has lived since 1999, because he didn't want to "stuck in a mass exodus if more radiation is released and if the winds blow towards Tokyo."
Americans aren't the only ones trying to leave Japan.
"I don't worry about the earthquakes. I mean, I was on the 18th floor of Nissan's building, and we felt safe there, although stuff moves around quite a lot," Rynhardt Rall, a South African said as he leaving Japan from Narita Airport
Rall said he did not trust what he was being told by Japanese officials.
"I think the Japanese people, or the government let's say, has a hidden investment in nuclear. They want to tell people it's safe. I personally feel that when the stuff gets in the air, and the wind blows it around, I don't know which size of an exclusion zone would be safe. But I would rather be with the U.S. and say make it larger than smaller," he said.
While foreigners rush to the airport, few Japanese are leaving the country.
A Tokyo cab driver told ABC News, "I've taken about four trips to Narita a day for foreigners looking to leave the country. I have yet to come across a Japanese family that is trying to get out."