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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 after Friday’s massive earthquake and tsunami.

“I will not from here judge the Japanese evaluation of the data,” Mr. Carney told reporters. “This is what we would do if this incident were happening in the United States.”