Radiation from the crippled Soviet reactor in Chernobyl has reached the United States and is sweeping rapidly across the country at high altitudes, U.S. officials reported yesterday.

They said that there was no evidence that Americans are in any danger from the radiation.

Aircraft flying off the northwest U.S. coast early Sunday morning detected radiation levels "slightly above" what is considered normal at altitudes up to 30,000 feet, according to members of an interagency task force monitoring the impact of the nuclear disaster.

Somewhat higher levels of radioactivity were detected over the Gulf of Alaska, in a slower-moving air mass at 18,000 feet, and officials said overall radiation levels may increase when that mass arrives later this week.

Environmental Protection Agency Administrator Lee M. Thomas, head of the task force, repeated that there was no evidence of any health threat. Readings off the California and Oregon coasts are a small fraction of the radiation levels measured off the coast of Norway late last week, and U.S. officials did not believe the Norwegian levels were dangerous.

Nonetheless, Thomas said his agency had stepped up surface water and rainwater testing and would begin sampling milk every other day rather than weekly.

Detection of radioactivity at high altitudes means that some radioactive emissions have been lifted upward into the jet stream, which moves air from west to east across the Pacific Ocean at speeds up to 150 miles per hour. While the low readings suggest that the particles are widely dispersed, a severe rainstorm can wash the particles out of the air and deposit them on the ground in higher concentrations.

"The most likely source of early detection near the ground will be in rainwater, particularly from thunderstorms reaching altitudes of 20,000 to 30,000 feet or more," the task force said.

Lester Machta, director of the National Oceanic and Atmospheric Administration's air resources laboratory, said rain clouds on the West Coast yesterday were reaching to 20,000 feet, and severe thunderstorms in the Midwest were reaching to 45,000 feet.

"My guess is that radioactivity will be detected there," he said.

Thomas said, however, that it was not possible to guess where radioactive particles would fall. "It's virtually impossible to do any kinds of predictions before you begin to get some readings," he said.

Nonetheless, Thomas said that he did not expect it would be necessary to issue any advisories on drinking water, milk or fresh vegetables as has been done in some European countries.

Task force members sidestepped criticism of European officials, however, suggesting that nations with ample food may have restricted consumption for the sake of prudence.

"With radiation, unless you get some benefit from it, the best thing is to avoid it," said Sheldon Meyers, head of EPA's radiation programs.

Meyers said task force officials had made no decisions on what levels of radiation will be deemed acceptable in the United States. "We just haven't focused on it," he said. "We just learned, literally, that the cloud had arrived."

Government scientists measured 2.5 picocuries per cubic meter of air at 30,000 feet about 150 miles off the California-Oregon coast. A picocurie is a measurement of radioactivity that counts the number of atomic disintegrations per second. Machta described that reading as "just barely above background levels." Less than one picocurie per cubic meter would normally be expected at that altitude.

At 18,000 feet about 400 miles off Canada in the Gulf of Alaska, measurements were 15 picocuries per cubic meter. Levels of 600 picocuries were detected off the Norwegian coast last week.

EPA drinking water standards permit 15 picocuries per liter, and Meyers said it would necessary to drink water containing about 45,000 picocuries of radioactivity to exceed the acceptable annual dose of radiation.

Japan, he said, has measured rainwater at up to 10,000 picocuries per liter.

Picocuries measure only the amount of radioactive material, however, not the type of isotope emitting the radiation. Some isotopes are considered more dangerous than others, because they are taken up readily by living things or because they remain dangerously radioactive for months and years.

Machta said the task force had some data on the types of isotopes in the air mass sweeping over the United States, but the data had not been checked for accuracy.