U.S. Navy officials said yesterday that the search for two black boxes containing in-flight data recorders of the downed South Korean jetliner has been narrowed to a 15-square-mile area about 20 miles west-northwest of the tiny island of Moneron in the Sea of Japan.

At one point more than a week ago the search area had covered 3,000 square miles.

Navy personnel also said officials of the Montreal-based International Civil Aviation Organization (ICAO) had arrived in the region and were taken aboard U.S. Navy search vessels as impartial observers to witness recovery operations and confirm that the black boxes, if found, have not been tampered with.

Navy and State Department officials said, however, that the jumbo-jet wreckage containing the recorders had not been pinpointed. They denied recurring reports in the Japanese press that U.S. Navy vessels had located the black boxes.

American and Soviet vessels, equipped with underwater listening devices and unmanned submersible vehicles, have been combing a wide area near the Soviet island of Sakhalin. The recorders may contain clues to why Korean Air Lines Flight 007 strayed hundreds of miles into Soviet airspace before being shot down by Soviet jet fighters on Sept. 1.

U.S. officials said they believe that the current search area, which is in international waters, is the most likely place for the black boxes to be found. This is based on signals picked up for about 90 minutes earlier this month from an automatic beeper in the black boxes. Those signals were said to be lost, however, before the location could be pinpointed.

Since then, officials said, a few intermittent sounds have been detected. The beeper is supposed to last only about 30 days. However, officials said sonar techniques could be used to find the large tail section containing the black box, although the task is easier with the beeper.

The sea bottom in the area is said to be mountainous, with depths ranging from 300 feet to 5,000 feet, a factor said to be complicating the search greatly.