American crews on vessels searching the Sea of Japan for wreckage of Korean Air Lines Flight 007 have heard electronic beeps emitted by the "black box" housing the plane's in-flight data recorder, informed sources said yesterday.

The sources said the strong assumption is that the Soviets, who are scouring the area with dozens of ships and planes, also have heard the telltale beeps. But as of midday yesterday, the sources said, neither side appeared to have found the data recorder.

The electronic beeper, with which all airliners are equipped, is meant to help searchers find wreckage. The beepers normally operate for about 30 days. The South Korean airliner, with 269 people aboard, was shot down on the night of Aug. 31-Sept. 1.

The sources said "it is almost certain" that the wreckage containing the in-flight recorder is in international waters, rather than Soviet waters. As one official put it yesterday, "There is one helluva race going on out there" to locate the beeper and with it the crucial black box.

American, Japanese and South Korean officials say they believe the in-flight recorder could contain an explanation of why the plane strayed hundreds of miles off course into Soviet airspace before heat-seeking missiles fired from a Soviet Su15 fighter sent it into the sea.

The recorder could contain additional communication by the KAL pilot or crew that might indicate if they had warning that they were in trouble before the missiles struck.

The Soviet Union continues to contend that the Boeing 747 was on an espionage mission to gather information about sensitive military installations, and that it gave off unusual electronic signals in flight. Yesterday, Moscow claimed that the timing of the flight was meant to coordinate the airliner's electronic espionage with other American intelligence-gathering craft, including a satellite.

The United States and South Korea have denied that the airliner was being used for espionage.

But officials here have speculated repeatedly in recent days that if the Soviets find the wreckage they will attempt to fabricate documents or tamper with the black box to substantiate their charges that the airliner was on a spy mission.

The search area is centered in the seas off the southwestern tip of the Soviet island of Sakhalin, near a smaller island called Moneron. The Japanese have reported sea depths of 600 to 900 feet in the region, but military officials here have put them at 900 to 1,200 feet.

Sources said that the United States and the Soviet Union both have equipment in the region capable of locating the beeper and retrieving the wreckage, but that the Soviets probably have an edge because they have been in the search area longer with more equipment and thus have covered more ground.

Sources said the searching is being done by such devices as side-looking sonar, which is pulled along underwater and uses reflected sound waves to locate objects.

The Soviets have two dozen vessels in the area and have been observed lowering small submersible vehicles from salvage ships."There is one helluva race going on out there" to locate the beeper and with it the crucial black box.

There have been reports that the Soviets have raised at least one sizable piece of wreckage.

U.S. officials said that the Soviets probably had retrieved something, but there was no indication as of yesterday that it was the crucial piece of the plane containing the black box.

American sources said there appeared to be several large pieces of wreckage, and that they seemed to be strewn over a wide area.