Obama was to address the crisis in a public statement late in the afternoon.

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 are working 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.” That evacuation zone is far wider than that established by Japan, which has called for a 12-mile zone and has told those within 20 miles to stay indoors.

Daniel B. Poneman, deputy secretary of energy, told the briefing that his agency agreed with the 50-mile zone. A “very dangerous situation” remains in Japan, he said.

Still, Jaczko said, “Basic physics and basic science tells us there really can’t be any harm to anyone here in the United States or Hawaii or any territories,” such as Guam, American Samoa or the Northern Marianas.

Jaczko said the U.S. recommendation for the 50-mile evacuation zone was based on the “possibility of scenarios that we haven’t seen yet.” He also said it was based on “prudent assumptions and prudent assessments about what could happen.” Yet Poneman said the Japanese recommendation also was prudent.

Asked what could be done to make sure that radiation from the world’s worst nuclear emergency in a quarter century would not harm the United States, Jaczko said: “We are really focused on making sure first and foremost that the plants in this country are safe.”

The officials spoke as 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 tests to measure how much radioactive material had been deposited on the ground in Japan. That data, Poneman said, was consistent with the recommendation for American citizens 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.”

Facts on the ground at the damaged nuclear plant are “genuinely complex and genuinely confusing,” the deputy energy secretary added.

Meanwhile, Obama visited the Japanese Embassy in Washington to sign a condolence book. White House aides said he wanted to show the nation’s commitment to standing by the Japanese during the time of crisis.

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 that Obama had taken the rare step of asking the NRC, which is an independent agency, to take into account what is happening in Japan and to apply lessons learned to the analysis of security and safety of reactors here. “The fact that the president has made that request himself only adds to the urgency of that mission,” Carney said.

Some critics have suggested the administration should do more to closely 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 on 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.

“We … have an independent regulatory agency whose sole mission is to constantly review and evaluate the safety and security of the reactors in the United States,” Carney said. The nation’s 104 nuclear reactors provide 20 percent of U.S. electricity.