WASHINGTON – US President Barack Obama vowed Thursday to stand by Japan as it recovers and rebuilds, but defended a decision to go beyond Tokyo's advice for evacuating Americans near damaged nuclear plants.

Obama offered heartfelt sympathy to Japan's people faced with triple challenges after a mammoth earthquake and tsunami badly damaged several nuclear power reactors, in an address from the White House Rose Garden.

He also assured Americans there was no reason to think harmful radiation from Japan could reach US shores and said he had ordered a "comprehensive review" of US domestic nuclear plants to learn the lessons from Japan.

"We will stand with the people of Japan as they contain this crisis, recover from this hardship, and rebuild their great nation," said Obama.

The president also explained why US officials had decided on Wednesday to advise American citizens within 50 miles (80 kilometers) of the crippled Fukushima nuclear plant to evacuate or seek shelter.

The US no-go zone was much wider than the 12-mile (20-kilometer) exclusion zone set up by Japanese authorities and raised questions over why the United States and its close ally were not on the same page.

"This decision was based upon a careful scientific evaluation and the guidelines that we would use to keep our citizens safe here in the United States, or anywhere in the world," Obama said.

"We do have a responsibility to take prudent and precautionary measures to educate those Americans who may be endangered by exposure to radiation if the situation deteriorates," he added.

The president, who explained the US action in a late-night telephone call to Japanese Prime Minister Naoto Kan, said conditions did not currently call for an evacuation beyond the 50-mile radius.

US officials declined to criticize the Japanese decision, but pointed out that different nations had unique regulatory approaches.

"The recommendation is ultimately a precautionary measure right now based on... some of the risks and challenges going forward in this situation," said Greg Jaczko, chairman of the US Nuclear Regulatory Commission.

"We think it's a prudent measure to take."

Obama also repeated that despite public anxiety, especially on the US west coast, there was no reason to believe harmful radiation from Japan to threaten American territory.

"I want to be very clear, we do not expect harmful levels of radiation to reach the United States, whether it is the west coast, Hawaii, Alaska or US territories in the Pacific," Obama said.

Jaczko, in an earlier White House briefing, explained that US evaluations had found no real risk of radiation traveling across the Pacific and posing a threat to Americans.

"The basic physics and basic science tells us that there really can?t be any risk or harm to anyone here in the United States or Hawaii or any of the other territories. So that?s something that we feel very comfortable with."

In his Rose Garden remarks, Obama appeared moved by the plight of a nation he first saw as a boy with his late mother, and to which he has made several trips as president.

"The Japanese people are not alone in this time of great trial and sorrow. Across the Pacific, they will find a hand of support extended from the United States as they get back on their feet," Obama said.

He reached for inspiration in citing the story of a four-month-old baby found alive by rescuers, days after being swept out of its parents arms by the tsunami sparked by the 9.0 magnitude earthquake a week ago.

"No one can say for certain just how she survived the water and the wreckage around her. There is a mystery in the course of human events."

US officials also said Thursday that a 39-strong US team had begun taking aerial radiation readings in Japan, on a fixed-wing aircraft and a helicopter, as part of their efforts to help the Japanese effort to calm the crisis.

Preliminary readings had been taken after the United States issued its 50-mile radius recommendation but had showed the move was a prudent step, officials said.