FBI Director William H. Webster said yesterday that if Soviet KGB official Vitaly Yurchenko staged his July defection to embarrass this country before the Reagan-Gorbachev summit, it was "an act of folly" for him to give the United States so much valuable intelligence.

Webster, in an interview on ABC News' "This Week With David Brinkley," said Yurchenko had helped the Justice Department open a "substantial" number of spying investigations and reopen others.

Yurchenko -- a colonel in the KGB, the Soviet secret police, with a high position in the department responsible for intelligence operations against the United States and Canada -- announced his intention to return to the Soviet Union at a dramatic news conference on Nov. 4 at the Soviet Embassy here. He said he had been kidnaped and drugged by the Central Intelligence Agency, an allegation denied by U.S. officials.

Webster confirmed government officials' private assertions in recent days that Yurchenko had alerted authorities to at least two alleged Soviet spies: Edward Lee Howard, a former CIA trainee who allegedly told the Soviets about a U.S. agent in the Soviet Union before being unmasked and disappearing from his New Mexico home; and Ronald William Pelton, a former communications specialist with the National Security Agency charged last week with selling secrets to the Soviets.

Asked whether Yurchenko might have been a Soviet double agent who was trying to gain U.S. officials' trust by giving them the identities of some inactive former Soviet agents, while not harming active Soviet intelligence efforts, Webster said, "That analysis is ongoing, and I don't think we should close our eyes to that possibility.

"But certainly," Webster continued, "everything I know about it is that it would be an act of folly to have given up that kind information simply to have some embarrassment going on at the time of the summit.

"We have opened a substantial number of cases based on very useful information he has supplied," he said. "Not only new cases, but reviewing old information that might reflect on other security holes that were open in prior years."

U.S. officials are debating whether Yurchenko was a phony defector or was a bona fide defector who became depressed and decided to go home.

Bill Baker, the Federal Bureau of Investigation's assistant director for public affairs, said yesterday that Webster would not elaborate on his televised comments.

Webster reiterated that FBI counterintelligence agents are stretched to their limits in trying to keep track of the approximately 2,500 Soviet-bloc diplomats and consular officials in this country.

He said Soviet students who come to the United States under a Geneva summit agreement will probably include spies.

Sen. Patrick J. Leahy (D-Vt.), vice chairman of the Senate Select Committee on Intelligence, also expressed concern about the number of Soviet-bloc officials here. On NBC News' "Meet the Press," Leahy said yesterday that the State Department is "lobbying heavily" against implementation of a law, originally proposed by Leahy and Sen. William S. Cohen (R-Maine), to eliminate the Soviets' longstanding advantage in numbers of diplomatic officials here compared with U.S. officials in the Soviet Union.

Leahy said there is "almost a war going on between the State Department and the rest of the government about how they should implement the law." State Department officials have said they fear the measure would lead to Soviet expulsion of U.S. diplomats.

The comments of Webster and Leahy came amid revelations in the last two weeks about new spy arrests. Besides Pelton, the others arrested are Larry Wu-Tai Chin, 63, a retired CIA analyst who allegedly has been spying for China since the early 1950s; Jonathan Jay Pollard, 31, a civilian Navy counterterrorism expert who allegedly sold classified information to the Israeli government, and Pollard's wife, Anne Henderson-Pollard, 25, charged with unauthorized possession of classified documents.