Local officials at the Chernobyl nuclear power station initially failed to give Moscow a "true assessment" of the April 26 accident, a Soviet official told a press conference today, and mass evacuation of the population in surrounding areas did not begin until 36 hours later.

Deputy Prime Minister Boris Shcherbina, head of the special commission named to investigate the accident, said at the first press conference by Soviet officials on the incident of 11 days ago that the most likely cause was a "chemical explosion," which occurred "during a planned stop" of the station's No. 4 reactor. He called the accident the result of "the coincidence of several highly improbable and therefore unforeseen failures."

The press conference, conducted at the Foreign Ministry with representatives from scientific, health and environmental agencies, left many questions about the incident unanswered. Several officials read answers to written questions and only four queries were taken orally, all from Communist Party newspapers.

[In Washington, officials of a U.S. task force monitoring the accident called the new details "useful information," but said the Soviets still have given no clues to the initial cause of the explosion or when the plant began spewing radioactive emissions.]

The 68-minute press conference, which was shown in full on Soviet television this evening, broadened the scope of the accident for Soviet viewers, who were told for the first time that radiation was a matter of concern in the Soviet republic of Moldavia and in the East European countries of Hungary and Romania.

The Soviet media have not reported that the radioactive leak from Chernobyl released emissions that went as far as Scandinavia and are now being tracked at high altitudes across the United States, Japan and Canada.

Shcherbina said today that specialists arrived at the Chernobyl site early last week to find that they had not been given a complete picture of the accident, which the Soviets first publicly acknowledged on April 28.

"The first information we obtained was not the same as we obtained when we were in the area. Local experts had not made a true assessment of the accident," said Shcherbina, whose appointment as head of the commission was announced April 28.

Shcherbina also gave the first official figures on radiation levels, which he said now registered at 10 to 15 milliroentgen per hour in "the immediate proximity" of the Chernobyl site. He said the figure had dropped two to three times, and that the high had been recorded on April 27, the day after the accident. Shcherbina did not give that figure.

Yuri Cedunov, first deputy chairman of the State Committee on the Environment, said that measurements in Kiev shortly after the accident showed radiation at normal background levels, but that a southward wind had pushed up levels there three days later to 0.2 milliroentgen per hour. He said similar readings had been made in southern Byelorussia, a Soviet republic.

Last Friday, Moscow party chief Boris Yeltsin said in a television interview in West Germany that the radiation level in the vicinity of the plant was 200 roentgen per hour, or 300 times the lethal dose. On Sunday, Yeltsin said the level had fallen to 150 roentgens per hour.

[The Ukrainian Health Ministry has advised people in the Kiev area to avoid eating leafy vegetables grown locally and to spend less time outside, Reuter reported.]

Moscow has shown normal background readings for radiation, Soviet officials said today. Preliminary findings by American specialists bear that out, a U.S. Embassy spokesman said.

Soviet officials today defended how the information flow was handled from Moscow, asserting that the Vienna-based International Atomic Energy Agency had been informed by cable "literally immediately" after officials of the Soviet Atomic Energy Committee learned of the accident.

[IAEA spokesman James Dagleish said the U.N. organization was told of the accident by telephone on April 29, three days after the explosion, and got a written report the next day, the Associated Press reported.]

Shcherbina said the accident began at 1:23 a.m. on April 26, "during a planned stop" of the reactor. Specialists here interpreted that as a regular cool-down, either for repairs or maintenance.

Shcherbina said the likeliest cause of the accident was a "chemical explosion in the reactor."

"In view of the fact that the design was in keeping with both Soviet and international standards and that there was strict control over the assembly and commissioning of the equipment, it can be said that the accident has been the result of the coincidence of several highly improbable and therefore unforeseen failures," he said.

Shcherbina said the other three reactors at the plant had been switched to "operational reserve" after the accident. He said two of the reactors can be put back into operation any time, but the third unit "will certainly require a thorough technical check."

Evgeny Vorobev, first deputy minister of health, today gave the first details on the cause of death of the two men, still unnamed, who were killed during the accident. One died from burns covering 80 percent of his body, while the other was fatally injured by debris falling during the accident, Vorobev said.

Vorobev said that a total of 204 people were hospitalized as a result of the accident, of whom 18 were considered in serious condition.

The Soviet Communist Party paper Pravda gave the first graphic account of the accident this morning.

Shcherbina today said that to control the fire, helicopters had dropped 220 tons of sand, metal and boron.

Shcherbina reported that evacuation of the population had begun on Sunday, April 27, and lasted from 2 p.m. until 4:20 p.m. His statement indicated that a full-scale response to the disaster was not mobilized until 36 hours after the accident took place.

Shcherbina backed this up later by saying that on the evening of April 27, more than 100 radiation victims were brought to Moscow where they are being given "necesary medical aid."

The Pravda account, however, indicated that evacuation of settlements around the plant -- including principally Pripyat, with a population of 25,000 located five kilometers from the nuclear plant, and Chernobyl, Poless and Ivankov -- was under way Saturday, April 26, as doctors in Kiev, 60 miles to the south, came to work.

The delayed response indicated by Shcherbina's account could be attributed both to the possible coverup by local officials alluded to by Shcherbina, and to the burst of high levels of radiation that peaked on April 27, analysts here noted.

The discrepancy between Shcherbina's remarks today and the Pravda article could either be a mistake, or it could mean that the evacuations were staggered -- by town and villages or by proximity to the plant.