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The United States on Wednesday 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 nonessential travel to any part of the country as unpredictable weather and wind conditions risked spreading radioactive contamination.

President Barack Obama placed a telephone call to Prime Minister Naoto Kan 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 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.

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.

Earlier in the day, the United States urged Americans to stay at least 50 miles from the plant — four times the distance recommended by the Japanese government.

The Pentagon said U.S. troops working on relief missions can go within 50 miles to the plant with approval. Spokesman Col. David Lapan said the U.S. would review requests from the Japanese for assistance that would require troops to move within that radius, though no approval for such movement had been given since the stricter guidelines were enacted.

The dire warning from American officials came on the heels of testimony from Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, who said that a deep pool holding uranium fuel at the Fukushima Dai-ichi facility sat empty of water needed to prevent releases of radiation.

That assessment, the first detailed comments by an American official about Fukushima Dai-ichi, provided an even more worrisome picture than the one provided by the government in Tokyo and gave millions in Japan on Thursday a heightened sense of concern as they try to determine how far and how fast radioactive material might spread.

With Japan's northeastern coastline demolished and fears of radiation growing, Emperor Akihito made rare public remarks Wednesday, saying he was "deeply concerned" about problems at Japan's nuclear plants.

Failed attempts by Japanese officials to bring the reactors under control, coupled with the U.S. analysis of the situation inside the facility, suggested a greater likelihood that high levels of radiation are leaking from the plant.

In the latest effort to contain the growing problems at the plant, a Boeing C-47 helicopter began dropping water on the unstable unit 3 reactor at Thursday.

NHK reported there's an "urgent need for cooling" the fuel rods, which requires their being covered in water. The rods in units 3 and 4 were believed to be exposed.

Wednesday's televised address by the emperor underscored the gravity of the moment and highlighted the myriad problems still plaguing the country nearly a week after the 9.0-magnitude earthquake and tsunami struck: a death toll that grows by the day, conflicting safety and evacuation information, growing distrust by locals and foreigners who call Japan home, a scarcity of gas, food and other resources, and the difficulty some aid workers have had delivering supplies.

The National Police Agency released updated numbers Wednesday afternoon: 4,314 people dead and 8,606 missing. But the toll of casualties is expected to reach far higher.

With lingering fears over the inability to stabilize the damaged power plants, Japan's collective fear and suffering has yet to ebb.

Japanese officials have called for a 12 1/2-mile evacuation zone around the coastal nuclear plant and asked that people between 12 1/2 and 19 miles away stay indoors. Their assessment did not change Wednesday, even though plumes of white steam billowed from unit 3 of the Fukushima Dai-ichi nuclear power plant.

NHK television footage Thursday morning showed relatively small amounts of steam rising from units 2, 3 and 4 at Fukushima Dai-ichi. The exact cause of the steam was not immediately clear.

In order for the workers to resume trying to cool the damaged reactors, Japan's health and welfare minister had to waive the nation's standard of radiation exposure, increasing the level of acceptable exposure from 100 millisieverts to 250 - five times the level allowed in the United States.

Japanese officials were working on a plan to deliver water and cool the reactors from the ground. In addition, a spokesman for Tokyo Electric Power, which owns the facility, said that a power line being laid to the Fukushima Dai-ichi plant to help restore the reactor cooling systems is almost complete and that engineers plan to test it "as soon as possible."