The Soviet Union said tonight that an accident at a major nuclear power plant in the Ukraine had damaged a reactor and caused unspecified casualties. The unexplained accident also sent a radioactive cloud hundreds of miles over Scandinavia.

The announcement, unusual for the Soviet Union, which rarely publicizes disasters, came several hours after four Scandinavian countries had reported detecting abnormally high levels of radioactivity in their atmosphere and Sweden's ambassador to Moscow began questioning Soviet officials.

The Soviet statement did not say whether there were deaths in the accident, which the news agency Tass said was at the Chernobyl power plant, 60 miles north of Kiev. It said only that "measures are being undertaken to eliminate the consequences of the accident. Aid is being given to those affected."

Residents of Kiev told United Press International by telephone that all bus service there had been stopped so the vehicles could be used to evacuate those in the disaster area. They said, however, that they had no information about casualties and had heard no explosion.

The Tass statement, read on the television evening news, said one of the plant's atomic reactors was damaged and "a government commission has been set up" to investigate it. This was seen by western diplomats as an indication of high-level concern.

Some western diplomats here speculated that because of the proximity to Kiev, a city of 2.3 million people, and the unusual public announcement, there may have been a high death toll.

Officials in Sweden, where radioactivity was detected in the air, soil and tree leaves, said that the amount reaching that country was well above normal, but presented no immediate threat to the population.

Denmark, Finland and Norway also reported unusual radioactivity, with high readings coming from islands in the Baltic Sea.

Sweden said tonight that it is seeking "detailed information" from Soviet authorities about the accident, so that it can take precautionary measures if the contamination can be expected to continue or increase.

Swedish Energy Minister Birgitta Dahl said it was "unacceptable" that Swedish authorities and others outside the Soviet Union had been given no notification. Sweden would demand that the entire Soviet civilian nuclear program be made subject to international inspections, she added.

The radioactive cloud, traveling over the Arctic, could reach the West Coast of the United States in five or six days, a spokesman for the Environmental Protection Agency's Radiation Alert Network told United Press International. The spokesman said that until the agency could get a radiation-level reading, there was no way of knowing what effect the fallout could have in the United States.

White House spokesman Edward Djerejian said that the accident "must be very serious if the Soviets talk about it."

One western diplomat said the Soviet statement "almost certainly indicated that the death toll was high," UPI reported. Another said that, because of the wording of the statement, "it is not unreasonable to speculate about deaths."

The Soviets did not say when the accident occurred, but Scandinavian experts said that their trackings and wind patterns indicated that it had happened early in the weekend.

A later Tass story called the accident "the first one in the Soviet Union" -- a description disputed by U.S. and other western authorities -- and listed what it said were similar nuclear plant accidents in other countries, including the United States.

Although the official Soviet record shows no previous accidents here, there have been reports of several, of which the most serious was an alleged explosion at a remote plutonium processing plant in the Urals in 1957, reported by former Soviet scientist Zhores Medvedev and by a 1980 study compiled by the Oak Ridge National Laboratory in Tennessee.

Medvedev, who now lives in London, wrote that an explosion of nuclear waste near the city of Kyshtym killed and injured thousands of people. The area has been closed since the time of the reported disaster, and some of the towns have been removed from Soviet maps.

Unofficial sources have reported other events, such as damage in 1981 to a steam generator at a plant in Rovno, also in the Ukraine. In 1982, a Soviet magazine appeared to confirm a leak from the power plant in Leningrad, which raised radiation levels in the Gulf of Finland and the Baltic Sea.

Clearly anticipating western news accounts scrutinizing the safety record of the Soviet nuclear power industry, Tass described what it called "the dangerous situation" at U.S. nuclear plants, which it said was due to "the poor quality of reactors" and lax safety measures.

The Soviet Union has traditionally provided little information -- and then only belatedly -- on accidents and natural disasters of all kinds, particularly in sensitive areas such as nuclear power.

Asked about safety on a television program last winter, Anatoly Mayorets, minister of power and electrification, said that "unequivocally" nuclear energy is "ecologically the purest source of electricity" for both workers and the environment.

Many Soviet nuclear plants are built close to centers of population.

The promptness of tonight's disclosure of the Chernobyl accident, observers said, could reflect either a new trend toward openness or response to the concern raised in Scandinavia by the radioactive cloud.

Since Mikhail Gorbachev became the Soviet leader in March 1985, the Soviet press has been urged to treat shortcomings, failures, problems, even calamities, with more candor.

After the statement on the 9 p.m. news -- which is viewed by millions of Soviet citizens -- a formal request for additonal information was made to the Soviet Foreign Ministry by the Swedish Embassy, which hopes to get some answers by Tuesday.

Without full and official data, it is difficult, if not impossible, to track the true safety record in the Soviet nuclear program.

For the first time in history, the Soviets agreed last year to allow international inspectors to visit two nuclear power plants. Inspectors from the Vienna-based International Atomic Energy Agency visited the plants last summer, including one in Voronezh, in central Russia.

The Soviet Union now has 15 atomic power plants, with more than 30 reactors. Its nuclear power program, now gearing up for a major push, is one of the world's largest, ranking behind only the United States and France in capacity.

Nuclear power now supplies about 9 percent of the country's electricity needs, but that dependence is to be pushed up to about 19 percent by 1990, and 30 percent by 2000, according to recently adopted plans.

Dependence on nuclear energy is particularly important in the European part of the Soviet Union, where there are few remaining natural energy resources and a concentration of industry.

The current five-year plan calls for nuclear energy production to increase from 170 billion kilowatt hours in 1985 to about 370 billion in 1990.

Previous five-year plans have set ambitious goals for the nuclear power program that have not been met. A major problem has been the production of standardized 1,000 megawatt units, which was slowed down by problems at the giant Atommash facility in Volgadonsk.

The Chernobyl plant, reportedly situated near the town on the Pripyat River, is made up of four reactors, of which the most recent went into operation three years ago. All four reactors at Chernobyl are 1,000 megawatts, the basic Soviet model.