WASHINGTON (AP) -- 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 Thursday.

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Passengers and cargo arriving from Japan are being screened for radiation "in an exercise of caution," Homeland Security Secretary Janet Napolitano said Thursday. She said no harmful levels of radiation had been detected.

President Barack Obama planned to make remarks on the crisis later in the day.

On Wednesday, 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.

Earlier Wednesday, the Obama administration urged the evacuation of Americans from a 50-mile radius of the stricken nuclear plant, raising questions about U.S. confidence in Tokyo's risk assessments. Japan's government was urging people within 20 miles to stay indoors if they could not evacuate.

White House spokesman Jay Carney 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. As late as Tuesday, Carney had said those recommendations were "the same that we would take in the situation."

But conditions at the nuclear plant continued to deteriorate, with surging radiation forcing Japan to order workers to temporarily withdraw. Obama met at the White House with Gregory Jaczko, chairman of the Nuclear Regulatory Commission, who recommended the wider evacuation zone.

During testimony on Capitol Hill Wednesday, Jaczko said anyone who gets close to the plant could face potentially lethal doses of radiation.

"We believe radiation levels are extremely high," he said.

State Department spokesman Mark Toner said the U.S. had consular personnel in the Miyagi and Ibaraki prefectures and was sending officials out to check on Americans.

"We have consular teams on the ground," Toner said. "Where they can, they are going door to door. They are going to hospitals. They are trying everything in their power to reach out and find American citizens."

The Pentagon said U.S. troops working on relief missions can get closer than 50 miles to the plant with approval. 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 Pentagon said troops are receiving anti-radiation pills before missions to areas where radiation exposure is likely.

"U.S. forces remain in Japan and the U.S. has full capability to fulfill our alliance commitments to defend Japan and maintain peace and security in the region," Lapan said.

With the arrival of three more ships to the massive humanitarian mission, there were 17,000 sailors and Marines afloat on 14 vessels in waters off Japan. Several thousand Army and Air Force service members already stationed at U.S. bases in Japan have also been mobilized for the relief efforts.

Airmen have been flying search and rescue missions and operating Global Hawk drones and U-2 reconnaissance planes to help the Japanese assess damage from the disasters. The operation is fraught with challenges - mainly, figuring out how to continue to provide help amid some low-level releases of radiation from the facility, which officials fear could be facing a meltdown.

Weather also temporarily hampered some relief plans Wednesday. Pilots couldn't fly helicopters off the deck of aircraft carrier USS Ronald Reagan until late afternoon because of poor visibility. The 7th Fleet said 15 flights with relief supplies were launched from the eight-ship carrier group, about half as many as the 29 flights reported the previous day to deliver food, water, blankets and other supplies.

Several water pumps and hoses were being sent from U.S. bases around Japan to help at Fukushima, where technicians were dousing the overheating nuclear reactors with seawater in a frantic effort to cool them. The U.S. had already sent two fire trucks to the area to be operated by Japanese firefighters, said Cmdr. Leslie Hull-Ryde, a Pentagon spokeswoman.