WASHINGTON — The United States has authorized the first evacuations of Americans out of Japan, taking a tougher stand on the deepening nuclear crisis and warning 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.

The U.S. is doing minute-by-minute analysis of the fast-moving situation, Secretary of State Hillary Rodham Clinton said today.

President Barack Obama placed a telephone call to Prime Minister Naoto Kan on Wednesday to discuss Japan’s efforts to recover from last week’s devastating earthquake and tsunami, and the nuclear crisis at the Fukushima Dai-chi plant. Obama promised Kan that the U.S. would offer constant support for its close friend and ally, and “expressed his extraordinary admiration for the character and resolve of the Japanese people,” the White House said.

But a hastily organized teleconference late Wednesday with officials from the State and Energy Departments underscored the administration’s concerns. The travel warning extends to U.S. citizens already in the country and urges them to consider leaving. The authorized departure offers voluntary evacuation to family members and dependents of U.S. personnel in Tokyo, Yokohama and Nagoya and affects some 600 people.

Senior State Department official Patrick Kennedy said chartered planes will be brought in to help private American citizens wishing to leave. People face less risk in southern Japan, but changing weather and wind conditions could raise radiation levels elsewhere in the coming days, he said.

“This is a very serious problem with widespread ramifications,” Clinton said during a visit to Tunisia. “There will be a continuing evaluation. This is … a minute-by- minute analysis and we’re doing everything we can to support the Japanese and their heroic efforts in dealing with this unfolding disaster.”

Pentagon spokesman Col. Dave Lapan said the military will coordinate departures for eligible Defense Department dependents.

The 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. Tokyo, which is about 170 miles from the stricken nuclear complex, has reported slightly elevated radiation levels, though Japanese officials have said the increase was too small to threaten the 39 million people in and around the capital.

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

But the alliance looked likely to be strained, with the U.S. taking more dramatic safety precautions than Japan and issuing dire warnings that contradicted Japan’s more upbeat assessments.

The French government and Czech military have evacuated some of their citizens from Japan on special flights and Britain is planning the same.

Still, the French Foreign Ministry said in a statement today that it “strongly advises against trips to Japan” and urges French citizens in the Tokyo region to head south or return to France.

Two French government planes are bringing home French people who want to leave Japan. One left Tokyo on Thursday for Seoul, with 241 people aboard.

Air France has also increased the capacity of its twice-daily flights in and out of Tokyo.

France’s nuclear manufacturer Areva and utility EDF sent a plane today with 100 tons of boric acid to help contain radiation leaks, 10,000 protective uniforms, 20,000 pairs of gloves and 3000 protective masks. France is a leading nuclear energy producer and has pledged to help Japanese authorities deal with their nuclear crisis.

Two Czech military planes landed in Prague this morning after evacuating 106 people from Japan, mostly Czechs but also several nationals of Poland, Slovakia, Bulgaria and Korea. Also onboard were 41 members of the Czech Philharmonic Orchestra that had been touring Japan since March 6, as well as 11 children.

Britain is advising its nationals in Tokyo and to the north of the city to consider leaving the area, and the Foreign Office has decided to organize charter flights from Tokyo to Hong Kong to supplement commercially available options for those wishing to leave Japan.

Some countries were more measured in their reactions.

“If you are in the larger Tokyo area, we urge you to consider to move away, southward or out. This is completely up to you,” Denmark’s Ambassador Franz-Michael Skjold Mellbin said today in a live webcast transmission from the Danish Embassy in Tokyo.

“The worst thing may actually be to join the panic situation and get outside, get stuck for three days outside Tokyo en route south and be exposed to unnecessary high levels of radiation.”

Skjold Mellbin said the Danish Embassy would remain in Tokyo even with several meltdowns.

Italy is considering a special evacuation flight for Italians in special need, such as pregnant women, families with young children, those who might be ill or who just couldn’t afford commercial flights. Switzerland is also considering charter flights if needed.

The Swedish government is working on a plan to help its citizens leave Japan, and is currently looking at air travel capacity.