Japanese officials disclosed tonight that Soviet military commands in the area north of Japan had changed the codes and frequencies of their aircrafts' radio transmissions as a result of the release of a Japanese tape that provided a minute-by-minute account of the shooting down of a Korean airliner.

As a result, they said, Japanese military intelligence is now able to monitor only about 60 percent as much of Soviet pilots' conversations as had been monitored before the shooting down of Korean Air Lines Flight 007 last week.

Japan had taken a calculated risk, they said, in making available the tape, which was played at the United Nations yesterday to support U.S. charges of a deliberate attack by Soviet fighters.

The sources said that the Japan Defense Agency initially opposed releasing the tapes because such a disclosure would warn the Soviets of the monitoring and enable them to change radio patterns.

The final decision to disclose the tapes, sources said, was made by Prime Minister Yasuhiro Nakasone and his chief Cabinet secretary, Masaharu Gotoda.

The tapes have been the key weapon in the Reagan adminstration's effort to pin the blame for the tragedy directly on the Soviets. They record the Soviet pilot describing how he approached the target and fired his missile.

Japanese officials also offered an explanation today of why radar operators did not warn the Korean Air Lines 747 that it was far off course before it was shot down near the Soviet island of Sakhalin.

They said the ill-fated airliner was not within direct radar observation of Tokyo civil air controllers during the flight and that military radar operators did not pick it up until shortly before it was shot down.

The explanation was given by Foreign Ministry sources hoping to counter Soviet suggestions that Japan had failed to warn the jumbo jet that it was far off course as it flew across Soviet airspace north of Japan.

The question had been raised by the Soviet ambassador to the United Nations, Oleg Troyanovsky, as part of his countercharge against allegations that a Soviet fighter had deliberately shot down an unarmed passenger jet. It had also been raised by other countries at the United Nations, they said.

The Soviet ambassador here was informed of the Japanese explanation tonight in an angry exchange with a Foreign Ministry official.

Japanese Foreign Ministry sources gave this account of why radar operators were not able to tell the jet was off course:

The plane was never actually in the radar zone covered by Tokyo controllers, whose only knowledge of its whereabouts was what the KAL pilot told them by radio. There was no way to verify the pilot's information by radar.

The plane did eventually come within the scope of a military radar tracking station located near Wakkanai in far northern Japan. But that was only "a few minutes" before the plane was shot down and there was no time to give warning.

The Associated Press reported from Wakkanai that the Maritime Safety Agency said the number of Soviet vessels operating where the airliner was shot down nearly doubled, raising speculation the ships may have found wreckage or bodies. The deputy director of the agency office in Wakkanai, Hiroshi Kishima, said 13 Soviet ships--nearly twice the number seen in the area before Wednesday--were observed searching waters northeast of Moneron Island.

According to Foreign Ministry sources, Director General Yoshiya Kato of the European division told Soviet Ambassador Vladimir Pavlov the Soviet response to date to Japanese inquiries had been "quite unsatisfactory" and directly accused him of lying about the circumstances of the downing.

The Japanese government as yet has not decided what measures it will take against the Soviet Union, officials said tonight.