The Pentagon charged yesterday that Soviet ships in the Sea of Japan are harassing American search vessels, sometimes in hazardous ways, in an effort to block the U.S. Navy from recovering the "black box" in-flight data recorders of the downed South Korean jetliner.

Crew members of U.S. salvage vessels equipped with underwater listening devices have several times in recent days picked up the pulsating beeper that was installed along with the in-flight recorders in the tail section of the 747 jumbo jet.

But Soviet vessels, on at least three occasions in the past five days, have pulled close alongside or crossed close in front of the two key U.S. salvage ships while they were under way, officials said. The American ships took evasive action to avoid collision and in the process broke their search patterns, officials reported.

On other occasions, Soviet vessels reportedly crossed close to the sterns of the U.S. ships at high speed, churning up the water and interfering with the underwater sound equipment they were towing.

The wreckage of the aircraft is believed to be strewn across a wide area of the Sea of Japan near the Soviet island of Sakhalin but in international waters, where U.S. officials estimate depths are 2,500 feet or more.

Beginning last weekend, Pentagon officials said yesterday, the two salvage ships, the USNS Narragansett and the USNS Conserver, began picking up intermittent signals from the beeper, of a kind installed on all airliners to aid searchers.

On Monday, the Narragansett got what officials said was "a strong fix" on the beeper for about an hour. Then the signal was lost. Later it was picked up steadily again for 30 minutes, then lost.

The search area covers about 3,000 square miles, officials said, and to find the beeper U.S. search vessels need to get fixes on it from three directions. The Navy ships are searching along straight lines and when the Soviets force them off those they create gaps in the search pattern.

Pentagon press spokesman Benjamin Welles told reporters yesterday that "there have been no confrontations as such" between ships of the two nations. "But there have been continuing instances where U.S. ships have maneuvered to avoid potentially hazardous navigational situations," he said.

"We believe every effort is being made to hamper our search in international waters. Our efforts out there have been impeded by Soviet units," Welles said, accusing Moscow of "a very considerable lack of cooperation."

Navy officials charged that the Soviets were not conforming to international law and the Rules of the Road at sea. Welles said the United States had complained to Soviet authorities and reminded Moscow of an agreement reached in the early 1970s between their two navies about proper conduct of military ships when in proximity in peacetime.

Welles and other Pentagon and Navy officials left little doubt that the pinger that was heard belonged to the downed airliner. "We're quite certain that what we've got is what we're looking for," Welles said of the signal.

The beeper aboard the downed airliner reportedly can be heard underwater by sound-detecting equipment at a distance of 2,000 to 4,000 yards. A beeper is normally good for about 30 days. The plane was shot down Sept. 1.

The U.S. salvage ship Conserver carries an unmanned submersible drone that is capable of descending to 6,000 feet. It carries lights and cameras, and is capable of lifting objects up to 350 pounds or of putting lines around heavier objects so they can be raised by surface vessels.

Officials here say they feel the United States has superior equipment and technology to locate and retrieve the wreckage. The harassing tactics by the Soviet ships may be meant to give their search vessels more time or they may reflect an inability to do much more than try to keep the U.S. Navy away from the wreckage.

Sources said the Soviets have about 20 vessels in the area, including small warships, salvage ships and intelligence ships, known as AGIs. It is these AGIs, officials said, that have been doing most of the harassing.

The United States has also beefed up its presence in the search area, officials said. In addition to the two salvage ships, a Coast Guard cutter and a frigate that have been there for several days, the replenishment ship USS Wichita, the guided missile cruiser USS Sterett and guided missile destroyer USS Callaghan are now on the scene.

The United States, South Korea and Japan hope the black box equipment will yield clues to why the Korean Air Lines plane strayed hundreds of miles off course and into Soviet airspace before it was shot down by Soviet fighters.

Actually, there are two black boxes, one containing flight information from aircraft instruments and the other containing the last 30 minutes of pilot voice communications and conversations in the cockpit. That recording might show whether the airliner pilots sensed they were in danger.

The Soviets have repeatedly alleged that the plane was on a spy mission, and there is concern here that if they get the black boxes, they will alter the data to back up their charges.

Meanwhile, the Soviets informed the American and Japanese embassies in Moscow yesterday that "they are prepared to transfer objects and documents recovered" from the downed plane to South Korea, with the United States and Japan acting as go-betweens. Moscow and Seoul do not have diplomatic relations.

State Department spokesman John Hughes described the latest Soviet allegations that the airliner was on a spying mission and that it was linked to an elaborate U.S. plan involving other spy planes, ships and satellites as "the kind of nonsense and lying that has characterized Soviet statements from the beginning."

"The United States does not use airliners for intelligence," Hughes said, adding, "The KAL plane was not involved in intelligence."