Radiation levels around the Soviet nuclear station near Chernobyl were high enough immediately after the accident to threaten the life of anyone within two to three miles of the reactor and severely endanger the health of people as far away as seven miles, U.S. officials said yesterday.

It was the first official estimate of the radioactive intensity of the Chernobyl accident. U.S. officials said they could not say with certainty how many people were harmed by the accident, because the Soviet Union has released no information on how quickly people near the site were evacuated.

"Whether anyone was under that plume depends on how much warning they had," said Nuclear Regulatory Commission official Harold Denton, a member of an interagency task force monitoring the impact of the world's worst known reactor accident.

In other developments, the task force said it could not confirm Soviet assertions that a fire in the reactor's graphite core had been extinguished. Officials said there was "some evidence" that the fire was still smoldering.

Lee M. Thomas, Environmental Protection Agency administrator and head of the task force, said that no unusual airborne radioactivity had been detected over Canada or the United States and that shifting winds may push the radioactive cloud back toward Western Europe, sparing the North American continent entirely.

"The longer the time, the less likely it is that we will see any detectable levels of radiation in the air mass over the North American continent," Thomas said. Officials continued to stress that no harmful effects are expected from the cloud, which will be much diluted if it reaches the United States.

The radiation levels near Chernobyl were calculated through extensive monitoring data from Sweden, where officials first reported abnormal radiation in the air Monday. Working backward toward the reactor and taking meteorological conditions into account, the task force said it thinks that "life-threatening" doses of radiation would have fallen within two to three miles of the plant.

Radiation levels would have dropped with distance. But even in areas from five to seven miles from the plant, there would have been enough radioactivity to cause "severe health effects," according to task force calculations. Those would include intestinal ailments, bone marrow destruction and damage to the body's immune system. Longer-term effects might include cancer, particularly thyroid cancer.

However, Denton said it is "entirely possible" that Soviet officials gave accurate casualty figures. The Soviets announced two deaths from the accident, and said 197 persons were treated in hospitals.

Denton noted that the casualty figure is close to the number of employes thought to be working at the Chernobyl plant. "Possibly they might have hospitalized everyone, but that's just sheer speculation on my part," he said.

Task force members said the radiation estimates support their belief that the Chernobyl plant suffered extensive damage to its fuel core, releasing "a large fraction" of the reactor's most dangerously radioactive materials.

"If the explosion was as violent as we think it was, a lot of that would have been ejected in the high atmosphere," Denton said. Immediately after the explosion, he said, radioactive debris would have spilled out of the reactor into the area surrounding the plant.

Officials said they are uncertain whether the reactor's 2,000-ton graphite block is still ablaze and emitting radioactive particles. "There is evidence that the reactor or associated equipment continues to smolder," the task force said.

If it still burns, however, the fire is not expected to add significant new amounts of radioactivity to the atmosphere. "The things that we worry about most have essentially all been released," Denton said. "Probably 90 percent of the isotopes that are important to public health and safety and environmental protection may have already been released."

An air mass containing those radioactive particles has spread over much of Europe and a large part of the Soviet Union, according to the task force. The United States has warned against travel to Kiev and adjacent areas and has warned children and women of child-bearing age to stay away from Poland.

State Department official James Devine said the advisory for Poland was issued because of "uncertainty about the situation."

Thomas said aircraft had detected radioactivity 400 miles west of Norway's coast at an altitude of 5,000 feet, but the mass is thought to be heading southward.

At current wind speeds and patterns, he said, "we could possibly, at the earliest, on Sunday or Monday see that air mass move over North America. It may be significantly later than that, if at all."