"The basic 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. It's really just based on the basic facts and science that's involved here," Nuclear Regulatory Commission chairman Gregory Jaczko told White House reporters at a news conference.

Earlier, a report in the Los Angeles Times said that adioactive isotopes are being blown toward North America "high in the atmosphere over the Pacific Ocean" and will reach California as soon as Friday.

US President Barack Obama also assured Americans that there was no danger of radiation for the country. "I want to be very clear: We do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska, or US territories in the Pacific," Obama said in Washington today.

"Let me repeat that: We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska, or US territories in the Pacific. That is the judgment of our Nuclear Regulatory Commission and many other experts."

The NRC Chairman, who has sent a team of about a dozen official and expert to Japan to keep a tab on minute to minute development, said the current situation in Japan with regard to nuclear radiation leak is very difficult.

"As we looked at a lot of the available information, we saw greater challenges, I think, with providing cooling to some of the spent fuel pools that had initially not been as much of an issue. So that was really one of the major changes that led us to reevaluate some of our information and come up with the recommendation we did," he said.

The Japanese are continuing to provide water into the spent fuel pools as well as continue to provide cooling to the reactor core. That's really what their focus is going to have to be for some time, is just to continue that activity of cooling and getting water or other means to cool the reactors and the pools.

"There clearly appears to be a challenge keeping that spent fuel filled with sufficient water. So it is a very dynamic situation," he said. Responding to questions on safety of nuclear installations, Jaczko stressed that the US has a very robust program where they look at the safety and the security of its nuclear facilities on a minute-by-minute basis.

"And this certainly will be new information that when we have good, credible information about what happened in Japan, we'll take that information and we'll work to see what changes we might need to make, if any, to our system," he said.