* "Don't panic" - naval base commander says
* Thousands of dependents could be affected - Pentagon
* Unclear which bases will be covered by Pentagon order (Adds figures on eligibility, paragraph 3)
By Phil Stewart
WASHINGTON, March 17 (Reuters) - The Pentagon on Thursday announced plans for a voluntary evacuation of U.S. military families from Japan's Honshu island amid growing anxiety about radiation leaking from a crippled nuclear power plant.
Pentagon spokesman Colonel Dave Lapan said the order would apply to thousands of dependents of U.S. military personnel at bases on Japan's biggest island, a massive undertaking that could involve the use of military aircraft if needed.
A spokesman for U.S. forces in Japan estimated that around 20,000 dependents would be eligible.
There are more than a dozen U.S. military bases and facilities on Honshu, including two naval bases where families were told this week to limit outdoor activity and shut off external ventilation after low-level radiation was detected.
"Don't panic," Captain Eric Gardner, the commanding officer at Naval Air Facility Atsugi, about 150 miles (241 km) from the Fukushima nuclear plant, said in a video-message for personnel.
"It (the voluntary evacuation) is unprecedented ... instead of the plan where those Americans in Korea would come to Japan, now we're just doing it in reverse."
The United States on Wednesday showed increasing alarm about Japan's nuclear crisis and urged its citizens to stay clear of the Fukushima nuclear plant, going further in its warnings than the Japanese government. [ID:nN16169392]
Even as the U.S. military ramps up a massive relief effort, it is also creating new restrictions meant to safeguard troops from the effects of radiation -- including by declaring a 50-mile (80-km) no-go zone for troops around the Fukushima plant and preventatively prescribing medication for radiation.
Some U.S. air crews took potassium iodide tablets ahead of missions that were within 70 miles (113 km) of the plant on Wednesday, the Pentagon said.
Potassium iodide tablets were also being shipped to Japan for troops, if needed.
Potassium iodide can saturate the thyroid gland and prevent the uptake of radioactive iodine. When given before or shortly after exposure, it can reduce risk of cancer in the long term.
Lapan said the voluntary evacuation would occur first by commercial or chartered aircraft, like those the U.S. State Department is using for its much smaller potential group of evacuees.
Gardner said Air Force planes would also fly out family members in what he described as a "military-assisted voluntary departure." But Lapan said they would only be used if there was sufficient demand among families.
Some of the U.S. bases on Honshu are well beyond the range of radiation, like the Marine Corps' Air Station Iwakuni, and it was unclear how many families would chose to participate in the voluntary evacuation.
"We're talking potentially thousands of people. It's all based on the volume," Lapan said.
"If there's a need for military aircraft then it will be there. But the first options are the existing commercial and charter aircraft capabilities." (Editing by Paul Simao and Eric Beech)