Even as the United States and Soviet Union explore a new era of peaceful relations, the Kremlin's spies pose as great a threat as ever to American security, the FBI says.

Soviet spies are using "every available platform" - from airline offices, news bureaus and trade delegations to the United Nations in New York and the Soviet Embassy in Washington - to acquire U.S. political, high-tech and military intelligence, FBI officials say.Record numbers of Soviet tourists, students, business leaders and emigres are traveling to the United States, thus increasing the pool of potential spies, say officials of the FBI, which oversees domestic counterintelligence.

"We're going to have a lot more Soviets in this country and a lot more potential for intelligence-gathering," cautioned James Geer, the FBI's assistant director for intelligence. "I have not seen any downturn in their activity and I don't expect to."

If Soviet leader Mikhail Gorbachev is to turn the Soviet economy around, Geer said, he will have to cut Soviet military funding for new weapons and knowhow, and "It falls to the KGB to fill the gap" with defense and strategy secrets from the West.

"Warming relationships do not signal a relaxing of the intelligence-gathering activities," Geer said.

CIA Director William Webster, through a spokesman, said that "Worldwide (the CIA) hasn't seen a decreasing of Soviet intelligence efforts during the period of glasnost. As a matter of fact, we believe their efforts to steal our technology are as strong as ever."

Roy Godson, a Georgetown University expert on Soviet intelligence and a former consultant to the National Security Council, said the KGB historically grows more active during periods of detente.

Godson said top-priority Soviet objectives continue to be high technology as well as information about political, military and economic policies that might affect the Soviet Union.

U.S. intelligence agents say the KGB puts high priority on U.S. military hardware, blueprints, product samples and test equipment. Targets include such things as microelectronics fabrication equipment and computers with military applications - all of which are barred for export to the Soviet Union.

Soviet interception of U.S. electronic communications and electronic penetration of computer systems pose major threats to U.S. military codes and other sensitive data, according to Pentagon intelligence experts.

On March 8 Lt. Col. Yuriy Nikolayevich Pakhtusov, a military attache to the Soviet Embassy here, was caught in an FBI sting operation attempting to steal U.S. security documents stored in computers. He was expelled from the country. Several weeks ago three West German computer hackers were arrested on charges they sold information to the Soviets that would help them gain access to top-secret U.S. computer banks.

"The Soviet electronic monitoring effort represents a significant worldwide threat to U.S. military and civil telecommunications," a Senate Intelligence Committee report says.

Congress isn't off-limits to Soviet spies, either. The KGB has repeatedly tried to elicit political and military secrets from Congress and to recruit or place agents in congressional offices, FBI officials say. Congressional telephones are among the KGB's special eavesdropping targets, according to the FBI.

Despite the era of "glasnost," Soviet front-group activities - those aimed at disinformation and identifying potential spy recruits - also are aggressive, Godson said. Last month, for example, the FBI arrested the director of a Soviet front group in New York for violating U.S. currency laws.

Alan Thomson, executive director of the New York City-based National Council of American-Soviet Friendship (NCASF), allegedly concealed the origin of two deposits totaling $17,000 made at a Buffalo, N.Y., bank, FBI spokesmen said. The money allegedly was paid to Thomson in Moscow by an official of the USSR Society for Friendship and Cultural Relations with Foreign Countries. Thomson has pleaded not guilty and is awaiting trial.

In 1987 the FBI identified the NCASF as a front group "largely financed and controlled" by the Soviet Communist Party Central Committee to promote propaganda and disinformation in the United States.

FBI officials say the NCASF has sponsored tours of the United States for Soviet tourists, and that KGB agents have used the tours as "cover" to visit American cities.

U.S. counterintelligence manpower and technology have vastly increased and improved in recent years, but the record number of Soviet emigres, tourists, diplomats, students and others in the United States makes it difficult to keep up with spies, Geer said.

"We can't run checks on 30,000 students or 20,000 visitors," he said. So the FBI focuses on known Soviet agents and their U.S. contacts.

In 1988, 21,512 Soviet citizens were issued tourist visas for virtually unrestricted travel in the United States, compared with only 2,350 six years ago.

Last year 25,000 Soviets were allowed to emigrate to the United States, but this year, according to Senate Foreign Relations Committee aides, more than double that number will be allowed.

"We're hearing the figure is 40,000 to 60,000," said a committee staffer. "That's an enormous increase. But we don't yet have an official figure."

Also, 2,000 people a month are expected to be admitted to the United States this year from Moscow on "humanitarian parole" - people allowed to join relatives in the United States who are in ill health or otherwise in need of the person.

Perestroika has brought a similar increase in diplomatic and trade delegates. Last year more than 270,000 foreign visitors entered the United States from the Soviet Union, Eastern Bloc countries, Cuba and the People's Republic of China, the State Department says. These visitors include 41,000 students and 5,800 diplomats and commercial personnel.

There are 243 Soviet diplomats assigned to the Soviet embassy in Washington, 28 in San Francisco, 150 at the U.N. Mission and 250 at the U.N. Secretariat.

The FBI believes one-third of the diplomats and commercial personnel are spies, as are many of the students.

There are about 70 U.S.-chartered corporations that, although owned by Eastern Bloc countries, function legally as U.S. corporations, subject to few restrictions on acquiring technology. U.S. officials say the primary interests of spies operating under commercial cover are economic data and advanced technology.

"Through their legitimate business activities, intelligence officers in those firms have access to Americans in business, industry and government who are potential targets for agent recruitment," according to a 1986 Senate Intelligence Committee report.

"A Czech, Pole or other East European is frequently able to contact U.S. companies without arousing the suspicion that contact by a Soviet official would occasion."

The U.S. security effort is ongoing, and this month the Senate and House Intelligence Committees will review counterespionage and counterintelligence strategies for the 1990s.

While the agenda is secret, the panels are expected to assess how successful U.S. intelligence agencies have been in reducing the amount of classified data and the numbers of workers with access to it.

The rash of spy cases in the past few years triggered an intensive effort to clear up backlogs of background investigations required for security clearances.

In Congress, control over secrets also has tightened, sources say. Fewer staffers have access to secrets, and sensitive documents are kept in vaults that meet higher security standards. And security training has increased.

The government also has expanded security training for both government and civilian workers with access to national secrets.

The FBI, for example, has stepped-up training under its "Development of CounterIntelligence Awareness" program for defense contractors. The program helps contractors safeguard data and detect foreign spies.

Meanwhile, U.S. intelligence agencies share training and information as never before, according to counterintelligence experts.