Soviet leader Mikhail Gorbachev, in his first public statement on the accident at the Chernobyl nuclear power plant, disclosed tonight that nine persons have died -- seven of them in hospitals after the accident -- and that 299 victims are now hospitalized.

The number of those in hospitals represents a sharp increase from the 204 persons reported as hospitalized only five days ago.

In a somber speech on national television, Gorbachev called the April 26 accident a painful "misfortune" that has been used in the West to mount "an unrestrained anti-Soviet campaign."

Gorbachev also used the occasion to extend a unilateral moratorium on nuclear testing until Aug. 6, the anniversary of the U.S. atomic bombing of Hiroshima, and to renew his offer to meet President Reagan -- in either Europe or Hiroshima -- for a start on test ban negotiations.

Most of Gorbachev's 26-minute speech was devoted to the Chernobyl disaster, which he said had revealed to the world for the first time "the sinister force" of "nuclear energy that has escaped control."

He described the cause of the accident as an unexplained surge that set off a hydrogen explosion while the reactor was being shut down for scheduled maintenance.

On April 29, the Soviets announced that two persons had died in the accident. On May 12, they said six had died of radiation and burns, but did not say whether the six were in addition to the two.

For the most part, Gorbachev stressed tonight, the danger is over. "Thanks to the effective measures taken, it is possible to say today that the worst has passed. The most serious consequences have been averted," he said.

"Of course, the end is not here yet. It is not the time to rest," he added, noting that radiation in the vicinity of the power station, 80 miles north of Kiev, the capital of the Ukraine, "still remains dangerous to human health."

Such a speech about a national disaster is considered unusual for a Soviet leader and in keeping with Gorbachev's campaign to shed some of the secrecy traditional to Soviet life.

Western diplomats tonight said its timing seemed to coincide with what one called a "public wind-down" of the incident.

Although Gorbachev's decision to address the nation was seen here as politically novel, even bold, some western diplomats expressed disappointment that he chose to attack the West's reaction to the crisis at Chernobyl.

"There was no suggestion that there had been any problems anywhere else in Europe," noted one western diplomat. "I had hoped he would have put things more on an even keel."

Some Western European governments have criticized the Soviet Union for not announcing the accident for 2 1/2 days -- until after radioactive fallout was detected in Scandinavia -- and for releasing few details in the first week.

More information has been made available over the past 10 days, as foreign and Soviet observers have gone to the site and reported back.

Gorbachev defended the Soviet handling of the accident, sharply denying charges that timely information had not been distributed to other countries. He said the charge was "invented," and claimed that the United States had taken months to inform the world about the nuclear accident at Three Mile Island, in Pennsylvania, in 1979.

But his harshest attack was for the reporting of the events at Chernobyl by "the mass media in certain NATO countries, especially the U.S.A.," which he described, "generally speaking," as "a veritable mountain of lies -- most dishonest and malicious lies."

Repeating themes that have been dominant recently in the Soviet press, Gorbachev said the exaggerations in the western press about death tolls, damages and panic among the population were the work of "certain western politicians" seeking to "sow new seeds of mistrust and suspicion toward socialist countries."

Gorbachev thanked foreign governments and individuals for offers of help, specifically mentioning the work of American doctors who have been assisting with treatment of radiation patients.

But he singled out the United States and West Germany as places where the accident had been manipulated for political ends, to divert attention from recent Soviet arms control initiatives.

Referring to the recent summit of industrialized nations in Tokyo, he said, "One involuntarily gets the impression that the leaders of the capitalist powers who gathered in Tokyo wanted to use Chernobyl as a pretext for distracting the attention of the world public from all those problems that make them uncomfortable."

"The accident at the Chernobyl station and the reaction to it have become a kind of a test of political morality," he said. "Once again two different approaches, two different lines of conduct were revealed for everyone to see."

In further extending the moratorium, Gorbachev seemed, according to observers, to be trying to deflect the reaction to the accident and to use it as another platform for a nuclear test ban, a prime component in the Soviet public disarmament program.

The Soviets unilaterally stopped testing last Aug. 6. The moratorium, due to end Jan. 1, was extended by Gorbachev until March 31, and again until the next test conducted by the United States, which took place last month.

At that time, the Soviet Union said it was forced to resume its own testing program, but commentators here declared the effort a "moral and political" victory.

Western diplomats here were skeptical of the latest offer, and of the renewed offer to meet Reagan, this time with Hiroshima as a suggested site.

Gorbachev tonight also proposed improvement in the reporting and monitoring of nuclear power plants, including a system of "prompt warning and supply of information" in case of accidents. He urged a further strengthening of the International Atomic Energy Agency, the U.N. affiliate whose general director, Hans Blix, recently toured the stricken plant at Chernobyl.

But Gorbachev firmly endorsed nuclear power, which plays an increasingly important role in meeting Soviet energy needs. "The future of the world economy can hardly be imagined without the development of atomic power," he said.

He promised that the Chernobyl accident would lead to measures that "rule out a repetition" of the event, and stressed that greater attention will be paid to the reliability of equipment, and "questions of discipline, order and organization" at nuclear plants.

Other Soviet officials and the press have blamed local officials for their slow response to the tragedy at Chernobyl, but Gorbachev tonight made no accusations, instead offering condolences to all affected, and praise to the rescue workers.

He also disclosed that a group on the ruling 12-member Politburo had been formed to monitor the disaster, headed by Prime Minister Nikolai Ryzhkov. The group is in addition to a special government commission.

His speech did not address the question of health hazards outside the area around the station, which has been an increasing concern among Soviets.

Gorbachev said that a hydrogen explosion had occurred when "the reactor's capacity suddenly increased during a scheduled shut-down of the fourth unit" at the plant. He said that the "considerable emission of steam and subsequent reaction resulted in the formation of hydrogen, its explosion, damage to the reactor and the associated radioactive release."

He said it is still too early to pinpoint the cause of the accident.

"All aspects of the problem -- design, projecting, technical and operational -- are under the close scrutiny of the government commission," he said.

Washington Post staff writer Richard Harwood reported from Vienna:

The Soviets have agreed to make a "full report" on the Chernobyl accident to the International Atomic Energy Agency in Vienna within two months, Director General Blix and nuclear chief Morris Rosen said in interviews. They said the report would include complete radiation readings from the Soviet Union.

In addition, the Soviets have agreed on measures such as post-accident reporting to other countries in future accidents.

The IAEA officials also said that the 204 casualties cited in previous Soviet reports were "heroic" volunteers who went into the plant area at the start to control the accident and put out the fire.

The radiation doses they suffered were moderate to very large, they said, and the injuries were from the "first degree" to the "fourth degree," with 18 having injuries of the "fourth degree." It was unclear what degrees the officials referred to, since radiation injuries are not usually categorized by degree of burns, but by the organ systems damaged.