The decision to shoot down a South Korean airliner over the Soviet island of Sakhalin last week was made by local air defense commanders convinced that it was on a spy mission for the United States, the Soviet Union's top career soldier insisted today in an unprecedented news conference.

The first detailed Soviet account of the disappearance of Korean Air Lines Flight 007 was provided here by Marshal Nikolai Ogarkov, chief of staff of the Soviet armed forces. Questioned closely by western journalists, he depicted the whole affair as a provocation engineered by Washington to blacken the Kremlin's reputation.

Some of the details offered by Ogarkov dovetailed with accounts by U.S. and Japanese officials. But there were also sharp discrepancies revolving around the Soviet insistence that the Korean plane was flying without navigation lights and ignored repeated warnings that it was intruding into Soviet airspace.

The detailed explanation given by the Kremlin more than a week after the downing of the plane with the loss of 269 lives seems unlikely to calm the furor that the incident has created around the world. The 63-year-old marshal and other senior officials at the news conference made clear that the Soviet Union was publicly unrepentant about the decision to shoot down the airliner, reserves the right to take similar action in the future and is not prepared to pay compensation to the victims.

NATO countries, meeting in Brussels, failed to agree on a joint response to the Soviet action. Details on Page A14.

"It has been proven irrefutably that the intrusion of the South Korean airlines plane into Soviet airspace was a deliberately, thoroughly planned intelligence operation," Ogarkov said.

The chief of staff confirmed that the South Korean plane had been shot down by two air-to-air missiles fired by a Sukhoi 15 interceptor jet. He said the decision to fire was made by the district command of the Soviet defense forces on the basis of strict standing regulations--and only later referred to Moscow.

Much of the two-hour press conference was taken up with a description of how the local air defense commanders came to the conclusion that the South Korean plane was on a reconnaissance mission. Ogarkov used a huge wall map of the Soviet Far East to show how the Boeing 747 jumbo jet had strayed by as much as 300 miles from the recognized international flight route from New York to Seoul in the early morning of Sept. 1.

He said the United States had failed to explain how the airliner managed to wander off course by such a huge distance while it was still within the visibility range of U.S. radar. He said Soviet radar first picked up the plane 480 miles northeast of Petropavlovsk-Kamchatski.

The wall map also showed the alleged route of an American RC135 reconnaissance plane which U.S. officials have acknowledged was off the northeast coast of Kamchatka peninsula at the time. Ogarkov said that the radar blips of the two planes merged at one point for 10 minutes, after which the RC135 flew back in the direction of Alaska while the South Korean plane continued toward the Soviet Union.

"Naturally the conclusion was reached at Soviet antiaircraft defense command posts that an intelligence plane is approaching Soviet airspace," he said.

At 16:30 Greenwich time, Aug. 31, according to Ogarkov, the South Korean plane approached Kamchatka and headed straight for a major Soviet nuclear base. Four Soviet MiG23s and SU15s were scrambled over Kamchatka between 16:37 and 17:08 Greenwich time, he said.

After ignoring radio signals to land at the nearest airfield, the jumbo jet then headed across the Sea of Okhotsk and on toward the island of Sakhalin, he said. Ogarkov said that six air defense planes were scrambled over Sakhalin and operated from 17:42 to 18:28.

The Soviet defense chief said that at this point "the actions of the intruder plane became outrageous." It sharply changed course at 18:02 Greenwich time, evaded positions of air defense missile units, and passed over important military installations at the southern end of the island, he said.

Ogarkov said that a final attempt to persuade the South Korean plane to land at an airfield was made at 18:20 Greenwich time on the southwestern corner of Sakhalin. A Soviet fighter plane fired four bursts of warning shots with 120 tracer shells but the jumbo jet attempted to escape toward the naval base of Vladivostok, he said.

The Soviet claim that warnings shots were fired at 18:20--just before the South Korean plane was downed--is totally at odds with a transcript of the conversation between Soviet pilots and ground stations that has been released by the United States. The transcript makes no reference to such warning shots.

Ogarkov said the order to down the "intruder plane" was given at 18:24 Greenwich time. According to the U.S. version, the pilot of the attack aircraft reported executing the launch and "destroying the target" two minutes later, at 18:26.

The packed news conference, held in the Foreign Ministry press center built for the 1980 Olympic Games, was open to all accredited journalists and diplomats in Moscow. Although journalists were encouraged to submit questions in writing, at least a dozen spontaneous questions were accepted from the floor.

Repeatedly asked by western reporters for proof that the South Korean plane was on an intelligence-gathering mission, Ogarkov cited its "totally incomprehensible behavior," its flight path over top secret military installations and "its curious relationship" with the RC135. He also repeated charges that it emitted bursts of coded intelligence data.

"What other proof do you need?" he asked, his voice rising in carefully controlled anger.

Ogarkov, who is also first deputy defense minister, said that it had been impossible to tell that the "intruder plane" was a civilian airliner. He said the incident had taken place during a dark, cloudy night and repeated claims that the silhouette of a Boeing 747 resembled that of an RC135, which is a military version of the smaller Boeing 707.

The chief of staff did not dispute the authenticity of the recordings of air-to-ground conversations released by the Reagan administration. Questioned about a passage in which one of the Soviet pilots reports seeing the airliner's strobe light flashing, Ogarkov said that the pilot had been talking about the lights of another Soviet interceptor ahead of him.

This statement is also directly at odds with the American version. According to the U.S. transcript, the pilot of the interceptor which fired the missiles is the one who talks about the flashing lights--and at one point he refers specifically to "the target's light."

Ogarkov refused to provide a Soviet version of the radio conversations, which he said had formed vital evidence for the commission set up to investigate the disaster. He said that members of the commission had also talked with many of the direct participants.

The marshal said that the intruder plane was hit over the settlement of Pravda on the southwestern corner of Sakhalin island and then followed a "descending flight" before hitting the sea at an unknown location. Soviet searchers had found some debris, he said, and there was a possibility that this might be handed over to the Japanese at an appropriate time.

Asked why it took six days for the Soviet Union to acknowledge officially that it had shot down the airliner, Communist Party spokesman Leonid Zamyatin insisted that the information had been contained in the first reports issued by the Soviet news agency Tass.

"We understand our own language better than you, particularly the political overtones," he said.

The initial Tass reports last Thursday and Friday did not mention the firing of missiles. By Friday, they acknowledged only that a Soviet interceptor had "fired warning shots . . . along the flying route of the plane," which then "continued its flight toward the Sea of Japan."

Zamyatin said today that the delay of over two hours before finally shooting down the plane was a testimony to the "humane character of Soviet air defense forces." Ogarkov added that it would have been quite easy to destroy the plane with SA5 rockets over Kamchatka.

Asked if the Soviet Union would pay compensation to the victims, Deputy Foreign Minister Georgi Kornienko said that "financial responsibility must be borne by those who sent the passengers to their deaths," meaning the United States.