The first U.S.-chartered aircraft left Japan Thursday with about 100 people aboard headed for Taiwan and another flight is anticipated on Friday, the U.S. State Department said.

Under Secretary of State Patrick Kennedy said the passengers included family members of U.S. government employees and a small number of private U.S. citizens who have chosen to depart Japan due to the ongoing nuclear crisis.

Thousands of family members of U.S. service personnel in Japan were offered flights out of the country Thursday as radiation continued to seep from a stricken nuclear plant.

The Pentagon said all families at bases on Japan's main island, Honshu, would be given the chance to leave.

For the U.S. military, which has more than 55,000 troops in and around Japan, as well as more than 43,000 dependents and thousands more civilian defense employees, the potential scale of the voluntary departure could be enormous.

"Don't panic," Capt, Eric Gardner, the commanding officer of Naval Air Facility Atsugi, about 150 miles from the Fukushima nuclear plant, said in a video-message for base personnel.

Gardner said the military would able to take about 10,000 people a day out of the country initially.

"Within the next 24 hours, there will be Air Force cargo, Air Force passenger planes landing here at Atsugi (air base), and taking first women and children out of here, most likely over to Korea for one or two days and then for further transfer to some other place," Gardner said.

The second phase of the plan would involve chartered commercial aircraft to fly out families.

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 no-go zone for troops around the Fukushima plant.

Atsugi was one of two U.S. naval bases that told personnel and families to limit time outdoors and to shut off external ventilation after detecting higher-than-normal levels of radiation.

The Pentagon also said some U.S. air crews started preventatively taking potassium iodide tablets on missions that were within 70 miles of the plant as a way to guard against effects of radiation.

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.

Late Wednesday, the U.S. State Department authorized the voluntary departure, including relocation to safe areas within Japan, of about 600 family members of diplomatic staff in Tokyo, Nagoya and Yokohama.

Diplomatic dependents have not been ordered to leave, but the State Department will bear the expense of their transportation if they choose to go, NBC News reported.

Commercial airlines were scrambling to fly thousands of passengers out of Tokyo Thursday.

On a smaller scale, Temple University in Pennsylvania said it was arranging a charter flight to evacuate 200 of its American students currently in Japan, University President Ann Weaver Hart said, according to a report in The Philadelphia Inquirer. The school will cover the cost of the flight.

The students were due to fly to the U.S. through Hong Kong on Saturday. Most of its non-U.S. staff and students, who are mostly Japanese, had decided to stay, Hart said.

The United States has urged its citizens in Japan to consider leaving and warned U.S. citizens to defer all non-essential travel to any part of the country as unpredictable weather and wind conditions risked spreading radioactive contamination.

Joanne Moore, a spokesperson for the U.S. State Department, told msnbc.com that the government did not know how many Americans were in Japan as they are not required to register with the embassy there.

However, according to the Japanese Bureau of Statistics website, there were 52,683 Americans registered as living in Japan as of Dec. 31, 2008.

The State Department has set up an e-mail address — japanemergencyusc@state.gov — at which Americans could seek help leaving the country.

Should a large-scale airborne evacuation begin, the airports there appear to have significant capacity.

Tokyo's Haneda Airport, the main international hub, was the fifth biggest in the world in 2009 in terms of passenger numbers, according to the Airports Council International, handling more than 61.9 million people that year, or just under 170,000 every day.

"Authorized departures" — in which the United States provides its own transportation to get Americans out of a dangerous situation — are relatively rare.

The U.S. government chartered flights to take Americans out of Egypt beginning Jan. 31 and ferries to remove them from Libya beginning Feb. 23.

But the exodus from Japan poses major questions that can't be answered by the experiences in those countries — most significantly whether the United States can conduct an orderly evacuation of so many people from a small and crowded country in the midst of a disaster.

Roughly the same number of Americans — about 50,0000 — were living in Egypt before the demonstrations that led to the ouster of President Hosni Mubarak last month.

But the evacuations in Egypt were centered mainly in Cairo, the capital, not the entire country.

The U.S. population in Libya, meanwhile, was only a few thousand before the popular uprising began there last month.

U.S. and Japan disagreeing?
President Barack Obama called Prime Minister Naoto Kan on Wednesday to discuss Japan's efforts to recover from last week's devastating earthquake and tsunami and the ongoing nuclear crisis at the Fukushima Dai-ichi plant .

Anxious to safeguard the U.S. relationship with its closest Asian ally, Obama told Kan about the steps the U.S. was taking, shortly before the State Department announced the first evacuations.

A message from U.S. Ambassador John Roos urged "as a precaution" that American citizens who live within 50 miles of the Fukushima complex to evacuate the area or to take shelter indoors if safe evacuation is not practical.

This contrasts with the 12-mile mandatory exclusion zone set up by Japan around the power plant. The government is also urging people within 20 miles to stay inside.

Hidehiko Nishiyama, deputy director-general of Japan's Nuclear and Industry Safety Agency, said Thursday that there were "two views" among Japanese officials about the state of the power plant's reactors.

Some agreed with a bleak assessment by top U.S. nuclear regulator Gregory Jaczko that a cooling pool at one reactor had run dry, but others thought it was still having an effect on overheated fuel rods.

Nishiyama said it was understandable that "an official of a foreign government would take the conservative view."

White House spokesman Jay Carney also sought to minimize any rift between the two allies, saying U.S. officials were making their recommendations based on their independent analysis of the data coming out of the region following Friday's massive earthquake and tsunami.

"I will not from here judge the Japanese evaluation of the data," Carney told reporters. "This is what we would do if this incident were happening in the United States."

Until Wednesday, the U.S. had advised its citizens to follow the recommendations of the Japanese government.

The U.S. decision to begin evacuations mirrors moves by countries such as Australia and Germany, who also advised their citizens to consider leaving Tokyo and other earthquake-affected areas.