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WASHINGTON — The first readings from U.S. data-collection flights over the stricken Fukushima Daiichi nuclear plant in Japan show that the worst of the contamination has not spewed beyond the 18-mile range of highest concern established by Japanese authorities, but there is also no indication that another day of frantic efforts to cool nuclear fuel in the reactors and spent fuel pools has yielded any progress, according U.S. government officials.

The data was collected in the first use of the Aerial Measurement System, among the most sophisticated devices rushed to Japan by the Obama administration in an effort to help contain a nuclear crisis that the top U.S. nuclear official said Thursday could go on for “possibly weeks.”

The data show ground-level fallout of harmful radioactive pollution in the immediate vicinity of the stricken plant — a different standard than the trace amounts of radioactive particles in an atmospheric plume now projected to cover a much broader area.

While the findings were reassuring in the short term, the United States declined to back away from its warning to Americans to stay at least 50 miles from the plant, a far larger perimeter than the Japanese government has established.

President Barack Obama affirmed the warning, saying in the Rose Garden on Thursday afternoon that the decision was based on “a careful scientific evaluation” of the “substantial risk” to those near the plant. He also repeated that there was no expectation that the radioactive plume emitted by the plant would bring harmful levels of radiation to any part of the United States, including its territories in the Pacific.

But he also called for a “comprehensive review” of the country’s nuclear plants.

In interviews, U.S. officials said their biggest worry about the Japanese plant was that a frenetic series of efforts by the Japanese military to get water into the four reactors there showed few signs of working.

“What you are seeing are desperate efforts — just throwing everything at it in hopes something will work,” said one U.S. official with significant experience in nuclear matters, who would not speak for attribution. “Right now this is more prayer than plan.”

After a day in which U.S. and Japanese officials had radically different assessments of the danger of what is spewing from the plants, the two governments attempted Thursday to join forces. Experts met in Tokyo to compare notes. The United States, with Japanese permission, began to put the intelligence-collection aircraft over the site in hopes of gaining a view for Washington as well as its allies in Tokyo that did not rely on the announcements of officials from the Tokyo Electric Power Co. Officials say they suspect that company has consistently underestimated the risk and moved too slowing to contain the damage.