US officials Wednesday warned citizens living within 50 miles (80 kilometers) of a crippled Japanese nuclear plant to evacuate or seek shelter, amid mounting concern of a catastrophic meltdown.

The evacuation order came as the chair of the US Nuclear Regulatory Commission (NRC) warned there was no water left in the spent fuel pool of reactor 4 at the Fukushima nuclear plant, resulting in "extremely high" radiation levels.

The US designated a much wider no-go zone than the 20-kilometer radius perimeter set up by the Japanese, after US officials reviewed scientific and technical data about the nuclear crisis following Friday's quake and tsunami.

"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 to take shelter indoors if safe evacuation is not practical," the US embassy in Tokyo said in a statement.

NRC chairman Gregory Jaczko told lawmakers that a fourth reactor unit at the nuclear complex was giving cause for concern following a hydrogen explosion there a few days ago.

"We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures."

The White House admitted the advice it was giving to its citizens went beyond the advice Japan had given, but declined to evaluate the Japanese data.

"This crisis is happening in Japan. Obviously, the Japanese government has the lead here with regards to dealing with the crisis and advising its people," said White House spokesman Jay Carney.

"There are a lot of Americans in Japan. When the data that we receive is analyzed independently, we are going to give advice based on our analysis."

At a hearing Wednesday, Jaczko told lawmakers that the NRC had recommended the wider evacuation zone.

"Based on the available information that we have that for a comparable situation in the United States, we would recommend an evacuation for a much larger radius than has currently been provided in Japan," he said.

Carney told reporters that the United States had informed Japan of its more conservative evacuation recommendation before it was released.

The embassy stressed, however, that "there are numerous factors in the aftermath of the earthquake and tsunami, including weather, wind direction and speed, and the nature of the reactor problem" which would affect the risk of any radioactive contamination within the zone.

The operator of the plant in Japan said it was "concentrating its efforts" on restoring the power supply to reactivate its crippled cooling systems.

"We cannot tell when but we want to restore the power source as soon as possible," Tokyo Electric Power Co. (TEPCO) spokesman Naohiro Omura told AFP.

Omura said TEPCO was preparing to utilize power lines from Tohoku Electric Power Co. and trying to repair its damaged electric transmission system to connect with unaffected lines.

US troops and pilots are also barred from venturing into the potentially radioactive area around the stricken Fukushima nuclear plant without specific orders, Pentagon spokesman Colonel Dave Lapan told reporters.

The Fukushima No.1 power plant has been hit by a series of explosions since the quake and tsunami knocked out reactor cooling systems.

The US military has delivered high-pressure water pumps and fire trucks for use at Fukushima, but has not been asked to join directly in the effort to contain the damage at the crippled atomic plant, Lapan said.

The military had also decided to give out anti-radiation pills as a precaution to some US air crews who were scheduled to fly within the "vicinity" of the crippled Fukushima plant, Lapan said.

Testing earlier showed 17 US helicopter crew members from the USS Ronald Reagan aircraft carrier had been contaminated with low levels of radiation. Out of those exposed, two were given potassium iodide pills, he said.

Meanwhile, the main US manufacturer of potassium iodide tablets said it has run out of stock after a massive demand generated by the crisis.

Anbex has been flooded with thousands of orders for its Iosat drug since Friday's earthquake and tsunami.

"The spike is enormous ... we were out of stock by Friday night," said Alan Morris. "The demand mostly is coming from the west coast of the US, but there are a significant number of inquiries, requests, orders coming from Japan, Korea, all over the Far East."

earlier related report
Radiation 'extremely high' at Japan reactor: US
Washington (AFP) March 16, 2011 - There is no water left in the spent fuel pool of reactor 4 at Japan's Fukushima nuclear plant, resulting in "extremely high" radiation levels, the chair of the US Nuclear Regulatory Commission warned Wednesday.

"In addition to the three reactors that were operating at the time of the incident, a fourth reactor is also right now under concern. This reactor was shut down at the time of the earthquake," said NRC chairman Gregory Jaczko.

"What we believe at this time is there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the spent fuel pool," he said, noting the explosion happened several days ago but its effects were cause for concern.

"We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures."

Jaczko briefed lawmakers about the latest developments as part of a House of Representatives hearing on the US energy budget, after meeting with President Barack Obama at the White House.

He said that if the United States were facing a similar situation, it would order a much larger evacuation zone than Japan has (12 miles, 20 kilometers), and so the US has called on Americans within 50 miles (80 kilometers) of the Japan nuclear plant area to leave.

"Recently the NRC made a recommendation that based on the available information that we have that for a comparable situation in the United States we would recommend an evacuation for a much larger radius than has currently been provided in Japan," he said.

"As a result of this recommendation the ambassador in Japan has issued a statement to American citizens that we believe it is appropriate to evacuate to a larger distance, up to approximately 50 miles."

France's Nuclear Safety Authority (ASN) earlier Wednesday described the No. 4 rod pool as "the major concern" in the crisis.

"The next 48 hours will be decisive," said Thierry Charles, director for factory, laboratory, transport and waste safety at the French Institute for Radiological Protection and Nuclear Safety (IRSN).

In an assessment issued at 0800 GMT on Wednesday, the IRSN said the water in the No. 4 pool was "boiling."

"Without water replenishment, the fuel-rod assemblies will start to be exposed in a few days. If the pool runs dry, this would eventually lead to the meltdown of the fuel.

"In such an event, the corresponding releases of radioactivity would be far higher than those that have occurred up till now."

The IRSN said that at around 2100 GMT on Tuesday, "responders were unable to approach the pool because of an excessive ambient dose" of radiation "of around 400 millisieverts per hour."

A single dose of 100 millisieverts is considered by some experts to increase the risk of several types of cancer.