Washington ? President Barack Obama, trying to reassure a worried nation, declared Thursday 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 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.