Plume of radiation

Gregory Jaczko, the top US nuclear regulator, cast doubt on efforts to cool overheating reactors, saying workers may be hit with "lethal doses" of radiation.

"It would be very difficult for emergency workers to get near the reactors," Jaczko said.

A United Nations forecast projects the radioactive plume from the Fukushima facility would reach the Aleutian Islands on Thursday and hit Southern California late on Friday, The New York Times reported.

The projection, calculated on Tuesday and obtained by the newspaper, gives no information about actual radiation levels, it said. Health and nuclear experts emphasize that radiation in the plume will be diluted as it travels and will have extremely minor health consequences in the United States, it reported.

The US military has ordered its forces to stay 50 miles away from the plant, the Pentagon said. There are at least 55,000 members of the US forces in Japan and offshore assisting the relief operation.

"All of us are heartbroken by the images of what's happening in Japan and we're reminded of how American leadership is critical to our closest allies," Obama said in Washington.

"Even if those allies are themselves economically advanced and powerful, there are moments where they need our help, and we're bound together by a common humanity."

Conflicting reports

The State Department's warning to US citizens was based on new information collected by the US Nuclear Regulatory Commission, Department of Energy and other US sources.

The United States is trying to deploy equipment in Japan that can detect radiation exposure at ground level, US Energy Secretary Steven Chu told a congressional hearing.

The detection system is part of equipment and 39 personnel from the Energy Department sent to Japan, he said. It has also provided equipment to monitor airborne radiation.

The United States is deploying more radiation monitors on Hawaii and other US islands even though it does not expect harmful levels of radiation to reach US soil, environmental regulators said.

Chu declined to tell lawmakers, when asked, whether he was satisfied with Japan's response so far to its nuclear crisis.

"I can't really say. I think we hear conflicting reports," Chu said. "This is one of the reasons why (the United States is) there with boots on the ground ... to know what is really happening."

Beyond the risk to workers at or near the damaged nuclear plant, one scientist, Dr. Ira Helfand, warned of possible widespread contamination of people and land.

"We need ... to focus on the radioactive isotopes being dispersed at some distance from the plant, because this is going to cause a whole different set of health problems," Helfand, past president of the anti-nuclear group Physicians for Social Responsibility, said in a telephone briefing.

The Obama administration has maintained its support for expanding US use of nuclear energy despite renewed fears about its safety after the events in Japan.

But Secretary of State Hillary Clinton said on Wednesday the crisis raised questions about the use of nuclear energy in the United States.

"What's happening in Japan raises questions about the costs and the risks associated with nuclear power but we have to answer those," she said in an interview with MSNBC in which she emphasized the need for a comprehensive US energy policy.

"We get 20 percent of our energy right now in the United States from nuclear power."