The Soviet Union was conducting experiments to check systems at Chernobyl's fourth nuclear reactor when a sudden surge of power touched off the explosion last month, a Soviet official said today.

Viktor Sidorenko, deputy chairman of the State Committee for Nuclear Safety, would not elaborate on the type of tests when he spoke to a group of foreign journalists.

In Vienna, the Soviet delegate to the International Atomic Energy Agency said that the death toll from the April 26 accident at the Chernobyl nuclear power plant in the Ukraine had risen to 15.

Soviet officials have said that the explosion happened when heat output of the reactor suddenly went from 6 or 7 percent to 50 percent of the plant's capacity in 10 seconds. The power had been reduced for a prolonged period in preparation for a routine shutdown, one official said earlier this week.

"We planned to hold some experiments, research work, when the reactor was on this level," Sidorenko said. "The accident took place at the stage of experimental research work."

"These experiments were connected with checking up of some systems at the station," he said.

Sidorenko, whose committee has one representative at each of the Soviet Union's 41 reactors, gave few new details on the accident.

He said he was informed of the accident three hours after it happened -- in the early morning hours of April 26 -- and that he was at the site by that afternoon.

Asked why representatives of his agency told a Swedish diplomat two days later -- shortly after 6 p.m. on April 28 -- that they had no information about a nuclear accident in the Soviet Union, he replied: "The people at their phones at the committee did not have that information. I myself was at Chernobyl."

Giving out information was "not the prerogative of our agency," he added later. "This information was given by competent government authorities who were authorized to do this."

The Soviet news agency Tass announced at 9 p.m. April 28 that an accident had taken place at Chernobyl.

As more information about the accident emerges from official Soviet sources, a key question is the cause of the sudden surge in power, first mentioned by Soviet leader Mikhail Gorbachev in a speech last Wednesday.

Sidorenko provided no clarification, saying it was still being investigated. Evgeny Velikhov, vice president of the Academy of Sciences and head of the cleanup effort at Chernobyl, is expected to give a public presentation next week.

Sidorenko said workers were continuing to dig around the damaged fourth reactor as part of the effort to seal off or "entomb" it.

An article in the newspaper Trud today said 400 people -- miners and subway construction workers -- are carrying out the tunneling operation.

In another Soviet newspaper, a ranking health official conceded that there had been shortcomings in the medical response to the disaster.

"Unfortunately, locally, there are a few specialists in the field of medical radiology, but the majority of medical workers are not well prepared in this regard and not well informed. This is one of the serious gaps in the training of people in our health system," Oleg Shchepin said in an interview with the Literary Gazette.

Shchepin said that the first task of health officials had been to identify those hurt in the accidents. The most serious cases went to Moscow because local hospitals could not handle them, he said. Then evacuations had to be arranged.

Although Soviet newspapers have described an orderly evacuation, an article in today's Communist Party paper Pravda, suggested there was some confusion. It mentioned cases of families separated during the evacuation who are still looking for relatives.

Pravda said local leaders are being called to task for failing to pass the test posed by the crisis.