Washington (CNN) -- The United States dramatically expanded its evacuation warnings near the troubled Japanese nuclear reactors Wednesday, telling Americans to get at least 50 miles away.

The earlier recommendation from both the U.S. and Japanese governments was to stay 12 miles away or to take shelter indoors if evacuation was not possible within an 18-mile radius.

The U.S. change came Wednesday afternoon in Washington but in the middle of the night in Tokyo. It was the first time since the March 11 earthquake and tsunami in Japan triggered the nuclear crisis that the United States and Japanese governments have differed on advising what their citizens should do.

Before Wednesday, the United States had urged American citizens in Japan to follow the recommendations of the Japanese government.

The new U.S. recommendation "suggests that the advice the Japanese government is giving based on the information it has is different than the advice we'd be giving if this incident happened in the United States," White House spokesman Jay Carney told reporters in Washington.

"Their standards are different from ours based on how far you should evacuate," he said, calling the situation in Japan "very fluid."

State Department spokesman Mark Toner echoed Carney, saying that the United States has confidence in Japanese authorities but wanted to put out the new information about the crisis based on the evaluation of U.S. experts and what they would propose in a similar crisis inside the United States.

"This is not any kind of judgment on what the Japanese authorities are telling their public," Toner said at a mid-afternoon briefing at the State Department. "This is based on what we would advise American citizens here to do in a similar situation. So we are compelled based on that estimate, based on that appraisal, to advise American citizens in Japan to do the same. '

The decision on widening the exclusion area lagged behind the U.S. military, which had kept military personnel from operating within 50 miles of the damaged nuclear plant "days ago," according to Pentagon spokesman Col. Dave Lapan. State Department officials could not explain why the military was concerned about U.S. military operating in the area but the State Department did not feel a need to warn American citizens until early Thursday in Japan.

Just before the evacuation recommendation had changed, the U.S. ambassador to Japan had told journalists that the United States was in full agreement with the Japanese.

"Our experts are in agreement with the response and measures taken by Japanese technicians, including their recommended 20-kilometer (12-mile) radius for evacuation and additional shelter-in-place recommendations out to 30 kilometers (18 miles)," Ambassador John Roos told journalists at a briefing in Tokyo Wednesday afternoon.

Despite the major change, Toner said the widening of the evacuation area should not be viewed as any shift of confidence in Japan. "We feel we are cooperating effectively with the Japanese government and that we are communicating," Toner said.

The State Department is coordinating with other governments about helping people leave the areas devastated by the earthquake and tsunami as well as by the crisis at the nuclear plants.

But despite earlier U.S. statements that there were 160,000 Americans in Japan, Toner said no specific numbers were available. And he would not predict whether the United States would take steps to bring members of its diplomatic team and families back to the U.S.

"We are looking at all contingencies but I have no more to say on that. Posture hasn't changed," Toner said.