TOKYO — The United States Thursday warned American citizens living within 50 miles (80 kilometres) of a crippled Japanese nuclear plant to evacuate or seek shelter, and barred US troops from the area.

The evacuation order, issued in a statement by the US embassy in Tokyo, comes after US authorities reviewed scientific and technical data about the nuclear emergency sparked by Friday's quake and tsunami.

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 zone "applies just to US forces," Lapan said, although the no-go area is much larger than the 20-kilometre area set up around the complex by Japanese authorities.

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

"We are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometres) 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 its statement.

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.

Lapan meanwhile could not say why the zone imposed by US authorities was larger than the 20-kilometer perimeter established by Japanese authorities.

He said US forces "can't go there routinely. They can be granted an exception to go into that area if they're required to do so."

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, he 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 Fukishima 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 17 crew members, two were given potassium iodide pills, he said.

The carrier, off the east coast of Japan, deployed further north a few days ago after sailing into radiation that blew towards the ship, Lapan added.

The country's devastating 9.0-magnitude earthquake Friday was part one of an unprecedented triple disaster, including a killer tsunami that pulverized Japan's northeastern coast and damaged atomic reactors which have spewed radiation into the air and sparked fears of a nuclear meltdown.