WASHINGTON  >> President Barack Obama, trying to reassure a worried nation, declared today that "harmful levels" of radiation from the Japanese nuclear disaster are not expected to reach the U.S., even as other officials conceded it could take weeks to bring the crippled nuclear complex under control.

The situation remains dangerous and complicated at the damaged Fukushima Dai-ichi reactors in northeastern Japan, U.S. officials said.

"We’ve seen an earthquake and tsunami render an unimaginable toll of death and destruction on one of our closest friends and allies in the world," Obama said in brief remarks at the White House after a visit to the Japanese Embassy to offer his condolences.

Obama said he knows that Americans are worried about potential risks from airborne radiation that could drift across the Pacific. "So I want to be very clear," he said. "We do not expect harmful levels of radiation to reach the United States, whether it’s the West Coast, Hawaii, Alaska or U.S. territories."

In Hawaii, Gov. Neil Abercrombie today issued a statement that echoed the president’s comments.

"I want to reassure residents and visitors that the Nuclear Regulatory Commission, the highest authority on radiation in the nation, has indicated Japan’s nuclear emergency presents no danger to Hawaii," he said. "Our state and county monitoring systems have not detected any increase in radiation levels, and based on all available information, state and federal experts do not anticipate any risk of harmful radiation exposure to our islands. We are open for business. Hawaii continues to be the world’s paradise.

The governor also said "residents do not need to take protective measures at this time."

"Our state Department of Health is working closely with state, county and federal agencies to monitor the situation on a minute-to-minute basis," he said.

Obama said he had asked the Nuclear Regulatory Commission to conduct a "comprehensive review" of the safety of all U.S. nuclear plants.

"When we see a crisis like the one in Japan, we have a responsibility to learn from this event and to draw from those lessons to ensure the safety and security of our people," Obama said. 

There are 104 nuclear reactors in the United States, providing roughly 20 percent of the nation’s electricity. "Nuclear energy is an important part of our own energy future," Obama said. 

A leading industry group agreed with the review.

"A review of our nuclear plants is an appropriate step after an event of this scale and we expect that the Nuclear Regulatory Commission will conduct its own assessment," said Marvin Fertel, president of the Nuclear Energy Institute. "The industry’s highest priority is the safe operation of 104 reactors in 31 states and we will incorporate lessons learned from this accident."

Meanwhile, the first evacuation flight of U.S. citizens left Japan, the State Department said.

In the U.S., Customs and Border Protection said there had been reports of radiation being detected from some cargo arriving from Japan at several airports, including ones in Chicago, Dallas and Seattle.

Radiation had not been detected in passengers or luggage. And none of the reported incidents involved harmful amounts.

Homeland Security Secretary Janet Napolitano said the agency was screening passengers and cargo for "even a blip of radiation."

Obama defended the recommendation of federal nuclear safety officials for a 50-mile evacuation zone around the crippled nuclear power plant for American troops and citizens in Japan, even though that is far larger than the zone spelled out by Japanese officials. 

"This decision was based on a careful scientific evaluation," Obama said. "Beyond this 50-mile radius, the risks do not currently call for an evacuation." 

At the same time, he said it was important to evacuate Americans "who may be endangered by exposure to radiation if the situation deteriorates."

Japanese officials have established a 12-mile evacuation zone and have said that people living  12 to 20 miles from the plant should stay inside. 

Gregory Jaczko, chairman of the Nuclear Regulatory Commission, told reporters at a White House briefing it could be some time before the crisis is brought under control as crews work to cool spent-fuel rods and get the damaged Japanese reactors under control. The activity could continue for days and "possibly weeks," Jaczko said.

He said the U.S. recommendation that American troops and citizens stay 50 miles away from the nuclear complex was "a prudent and precautionary measure to take." But he also said "basic physics" suggested there was little risk to anyone in the United States or its Pacific territories.

Daniel B. Poneman, deputy secretary of energy, told the briefing that a "very dangerous situation" remains in Japan. Information at the nuclear plant is "genuinely complex and genuinely confusing," he said.

As the officials spoke, Japanese emergency workers sought to regain control of the dangerously overheated nuclear complex, dousing it with water from police cannons, fire trucks and helicopters to cool nuclear fuel rods that were threatening to spray out more radiation.

The U.S. Energy Department said it had conducted two separate aerial tests to measure how much radioactive material had been deposited in Japan. Those data, Poneman said, were consistent with the recommendation for Americans to evacuate a 50-mile radius around the plant.

The U.S. officials declined to criticize the Japanese call for a smaller evacuation zone.

"We’re analyzing the information, and we’re sharing it with the Japanese," said Poneman. "The preliminary look has indicated that the measures that have been taken (by the Japanese) have been prudent ones. And we have no reason to question the assessment that has been made or the recommendation that has been made by the Japanese authorities."

At his visit to the Japanese Embassy, Obama signed a condolence book and said: "We feel a great urgency to provide assistance to those … who are suffering."

In the book he wrote, "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 this time of need."

"Because of the strength and wisdom of its people, we know that Japan will recover, and indeed will emerge stronger than ever," he wrote.

The crisis has been complicated by the spare and often contradictory information issued by the Japanese government and the Tokyo Electric Power Co., heightening a sense of uncertainty about what’s happening in the reactors.

"It’s not easy to get information from the site," said White House spokesman Jay Carney.

Carney said the fact that Obama had taken the rare step of asking the NRC —  an independent regulatory agency that is not under the president’s control —  to undertake a review of U.S. reactor safety in light of the Japanese disaster "only adds to the urgency of that mission." 

Representatives of the nuclear energy industry said Thursday that operators of U.S. reactors already had begun taking steps to better prepare for an emergency in this country.

While it will take some time to understand the true dimensions of the nuclear disaster in Japan, "we will learn from them, we will get that operating experience, we will apply it and try to make our units even safer than they are today," said Anthony Pietrangelo, senior vice president of the Nuclear Energy Institute, a Washington-based industry lobbying group. 

Some lawmakers, including Barbara Boxer, D-Calif., chairman of the Senate Environment and Public Works Committee, have suggested the administration should do more to re-examine the nation’s aging network of nuclear power plants with an eye toward making them more accident-proof. In other countries, China has said it would hold off on approving new nuclear plants, and Germany has said it would temporarily switch off seven aging reactors.

Earlier this week, European Union energy officials agreed to apply stress tests to plants across the 27-nation bloc. Spanish Prime Minister Jose Luis Rodriguez Zapatero commissioned studies to determine how vulnerable his country’s six nuclear plants are to earthquakes or flooding.

Carney, when asked why the United States was not taking the more stringent measures of some other countries, said Obama had "full confidence" the NRC was doing its job.