The death toll from the accident at the Chernobyl nuclear power plant has risen to 19, according to a senior scientist who was a key adviser to the cleanup team.

The figure was announced by Yevgeni Velikhov, vice president of the Soviet Academy of Sciences, at a press conference held one month after the accident. The figure later was reported by the Soviet news agency Tass.

It was the first official casualty update provided here since Soviet leader Mikhail Gorbachev said that nine people had died as of May 14.

In Vienna last week, a Soviet official said the count had reached 15, but that figure was not reported here.

Velikhov, who spent two weeks in Chernobyl directing the burying of the heavily damaged No. 4 reactor, said today there was still concern about possible contamination of ground water.

"I would not like to create an illusion that all questions are already solved," he said. "We will monitor all water, [particularly] in the fall and spring when more [runoff] water flows. We will take all possible precautions."

In general, however, he stressed that the situation was under control. "I don't think at present there are any serious surprises to come," he said.

Velikhov indicated for the first time that the Soviet Union is taking a fresh look at its nuclear energy program following the Chernobyl accident. "The Chernobyl event will influence and affect our decisions in future technical and administrative policies," he said.

At the same time, he reiterated the Soviet Union's commitment to its nuclear program, now poised to more than double its capacity in the next five years.

Velikhov, pressed by western reporters for detailed answers to several questions, gave no further specifics about the April 26 accident or its causes, which he said would be provided in a report by a specially appointed government commission that could take months.

"I don't want to go into details," he said. "It is very difficult to get a clear picture of what is taking place."

Velikhov indicated several times, however, that a series of "incorrect actions" could have triggered a chain of events, leading first to a sudden surge in power, a release of steam and finally a hydrogen explosion.

Soviet newspapers have given various versions of what occurred. Today, the Communist Party paper Pravda quoted an unnamed firefighter as saying that the first explosion was followed by others.

The firefighter described first hearing a burst of steam, which he said was not unusual and which he ignored. Later, after the explosions, he said he saw a black burning ball rise up to the roof of the machine room of the reactor.

Velikhov today referred to the conflicting versions and chided both the Soviet and foreign press for "exaggerated" accounts.

In particular, he said that the explosion had not been as powerful as some had indicated, noting that not all windows in the reactor building had been blown out.

Velikhov again described the rescue operation, saying the major concern had been whether the impact of the 5,000 tons of sand, boron and concrete dropped on the structure from the air would crush an underlying foundation and sink the reactor into the ground.

He said the main fire was extinguished within the first days, but that there was still concern that fuel continued to burn inside the reactor.

[Pravda said today that radiation levels around the damaged reactor are still far above normal, The Associated Press reported. The labor union daily Trud said "only those who are reliably protected with lead sheets and heavy armor are allowed to work in direct proximity to the reactor," according to AP.]

Velikhov said it was unreasonable to expect that the population from inside the 18-mile evacuated zone would return soon.

"In the reasonable future, they can hardly be expected to come back," he said. "I do not think we will take any chances."

Velikhov indicated that in some directions, the evacuated zone extends farther than 18 miles from the reactor.

The No. 3 reactor at the plant, said to have suffered some damage, may not be brought into operation again, Velikhov said today.

In the article in today's Pravda, Lev Voronin, deputy chairman of the Council of Ministers, said it would be several months before two other reactors at Chernobyl are back on line, but that it should happen before the end of the year.

Velikhov said another remaining problem was where to bury contaminated debris from the accident. He also said topsoil in the contaminated zone was being removed.