WASHINGTON (AP) — 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 Daiichi 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 U.S. 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.

The Pentagon said U.S. troops working on relief missions can get within 50 miles of 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.