Anxiety over Japan's damaged nuclear plant increased today as the United States' top nuclear regulator told Congress the situation was worse than reported by the Japanese government and that "extremely high" radiation levels could hamper containment efforts.

Evacuees at a center near Fukushima.

The American Embassy in Tokyo, meanwhile, recommended evacuation to U.S. citizens within 50 miles of the plant--an area much larger than the approximately 12-mile radius established by the Japanese. Still, the U.S. Nuclear Regulatory Commission maintained that there was little cause for worry about radioactive drift on the part of residents of Hawaii or the West Coast of the U.S.

According to various reports, Gregory Jaczko (pronounced YAZZ-koe), head of the NRC, told the House Energy and Commerce Committee today that a pool storing spent nuclear fuel at the Fukushima Daiichi Nuclear Power Station was now empty, or nearly so, leaving the fuel rods within to potentially catch fire and spew their radiation directly into the air. Jaczko also said a second pool might be nearing the same condition.

If his information is accurate and the spent fuel is not submerged and thus properly cooled, The New York Times reported, the situation could--at its worst--mean that so much radiation would be released that containment efforts would have to be abandoned altogether. That in turn could lead to complete meltdown and the spread of far more radioactivity. A helicopter was reportedly dumping water on the Fukushima facility on Thursday morning, local time, in an effort to address the crisis.

The Washington Post, meanwhile, reported that U.S. Ambassador John Roos said he thought Tokyo was still safe from radiation but that after initially following the guidance of the Japanese government, he issued a statement citing NRC recommendations.

"Consistent with the NRC guidelines that apply to such a situation in the United States, we are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometers) of the Fukushima Nuclear Power Plant evacuate the area or take shelter indoors if safe evacuation is not practical," Roos said in the statement.

The Post said the difference in the size of the restricted areas put forth by the U.S. and Japanese governments seemed to arise from differing standards on radiation exposure, rather than different views on how much radiation has escaped.

The Times, however, reported that American officials have said Japan's leadership has appeared to be overwhelmed and uncommunicative about the disaster and that it might not be getting clear information from the nuclear plant's operator.

In guidelines issued today, the NRC also provided assurances about the drift of the plume from the Fukushima plant.

"In response to nuclear emergencies, the NRC works with other U.S. agencies to monitor radioactive releases and predict their path," the commission said. "All the available information continues to indicate Hawaii, Alaska, the U.S. Territories, and the U.S. West Coast are not expected to experience any harmful levels of radioactivity."

The Times said Jaczko was asked during a White House briefing on Monday if a meltdown of one of the reactor cores at Fukushima would increase the chance of harmful radiation reaching Hawaii or the West Coast.

"I don't want to speculate on various scenarios," Jaczko reportedly said. "But based on the design and the distances involved, it is very unlikely that there would be any harmful impacts."

The Times also reported that, remembering Chernobyl, European nations were issuing statements. Germany's Federal Office for Radiation Protection reportedly called any threat from the plume negligible and said iodine pills were unnecessary.

Late Wednesday, The Post reported that the U.S. had offered to evacuate family members of State Department and Pentagon officials out of northern Japan, including Tokyo, but that the Tokyo embassy would remain open. The State Department is also offering U.S. citizens who wish to leave Japan travel to safe locations in Asia.

The NRC said the public can sign up for its latest news releases here, or for e-mail notifications about releases here.