The first US government-chartered evacuation flight has taken off from quake-hit Japan for Taiwan, amid rising fears over the country's stricken Fukushima nuclear power plant.
The plane carried 100 Americans, mostly relatives of government workers.
France and China are also to evacuate thousands, while the UK and Australia have advised citizens to leave Tokyo.
President Barack Obama said harmful radiation was not expected to reach the US or its Pacific territories.
"We have seen an earthquake and tsunami render an unimaginable toll of death and destruction on one of our closest friends and allies in the world," Mr Obama said on Thursday at the White House.
"And we've seen this powerful natural disaster cause even more catastrophe through its impact on nuclear reactors that bring peaceful energy to the people of Japan."
'Exhaustive review' pledge
Mr Obama also advised Americans not to take precautionary measures against radiation other than to stay informed; runs on potassium iodide pills, seen as a defence against radiation poisoning, have been reported in a number of countries.
And Mr Obama pledged an "exhaustive review" of the US nuclear power infrastructure.
"Nuclear energy is an important part of our own energy future," he said.
Mr Obama has assured Japan of America's ongoing support, and the US has sent food aid and military personnel to back Japanese efforts, and is sending nuclear and biological hazard specialists.
Meanwhile, Australian Foreign Minister Kevin Rudd advised Australians to leave Japan, China has moved thousands of people to Tokyo to prepare for their evacuation, and France has assigned two government planes to evacuate its nationals.
Most other countries have also advised their nationals to leave the north-eastern region of Japan or to leave the country altogether.
UK Foreign Office said that any Britons living in or north of Tokyo should "consider leaving", but officials have stressed there is no immediate risk to health.
US Under Secretary of State Patrick Kennedy said another US evacuation flight was scheduled to leave Japan on Friday, AFP news agency reported.
Customs screen
US customs agents will screen passengers and cargo from Japan for "even a blip of radiation", Homeland Security Secretary Janet Napolitano said on Thursday.
She called the screening an "exercise of caution".
The US state department warned Americans to delay travel to Japan, and said unpredictable weather might spread radiation in the areas north of Tokyo.
It has offered voluntary evacuations to about 600 family members of state department personnel in Tokyo, Nagoya and Yokohama.
The US military has 55,000 troops in the Japan region, who have about 43,000 dependants. Thousands of other government and private sector employees also reside there.
source : bbc.co.uk
