The terse jargon of Soviet fighter pilots as they stalked and shot down a South Korean jetliner in the early morning darkness last Thursday masks what some U.S. analysts say was "an incredibly stupid" decision made by Soviet commanders who knew that the plane was rapidly heading out of Soviet airspace at 600 mph.

In other words, the Soviets had a crucial period of minutes to decide to down the plane before it would clearly have left Soviet airspace and been over international waters.

In 1978, Soviet fighter pilots also intercepted, fired automatic weapons at and ultimately forced down another South Korean airliner that had strayed deep inside Soviet territory. In that case, the Korean airliner that went off course was constantly over Soviet territory.

But in the incident that unfolded last week, Korean Air Lines Flight 007 first crossed into Soviet territory over the Kamchatka Peninsula for a short period. Then, apparently unaware that it was on the wrong flight path, it continued over the Sea of Okhotsk, which is international airspace. But the plane's course then took it over Soviet territory again over the island of Sakhalin where it was shot down just as it was leaving for what would have once again been international airspace.

Thus, while Soviet ground radars had been tracking the plane for about 2 or 2 1/2 hours, according to American accounts, it appears from the information released thus far that it was only in the last 21 minutes in the life of KAL 007 that the Soviets had fighters on the tail of the jetliner in a position to fire with an unseen airspace border approaching rapidly.

The presence of the fighters to identify a target is crucial because ground radars cannot tell with any precision what kind of plane is being tracked.

The recordings indicate that 15 minutes before they launched their missiles, the Soviet pilots saw a plane with wingtip navigation lights on and a "blinking" light that presumably was a strobe light that is carried on commercial jetliners.

This should have alerted the fighter pilots and the ground commanders that this was an airliner. But the recordings also show that the Soviet fighter pilots never came closer than two kilometers (1.2 miles) to their "target." While there is some debate about this, many aviators say it is difficult if not impossible to really identify the type of craft and certainly its markings at that distance.

The Soviets appear to have made no close-up attempt to make sure they knew what they were firing at. Yet the plane was heading out of Soviet airspace. Some officials speculate that the Soviets have standing orders to shoot down intruders after various procedures are followed and were probably mindful of the 1978 incident.

After the 1978 incident, which humiliated the Soviet air defense command because the airliner penetrated so deeply into Soviet airspace before it was intercepted, orders were reportedly tightened and it was also reported that some officers were executed or otherwise punished.

Thus, some officials here speculated that with time pressing them, the Soviets decided that nothing would happen to them if they shot down the plane, no matter what it was. It was an intruder in their airspace in any case and if it was a U.S. reconnaissance plane that they allowed to get away, the results for the air defenders could have been severe.

A statement by the Reagan administration yesterday said the incident "raises the most serious questions about the competence of the Soviet air defense system, with all the danger that implies."

The key part of transcripts released yesterday covers the 21 minutes between the time the two fighter pilots first reported seeing a target, presumably its lights, and the time the jetliner was shot down. These two planes, an Su15 that fired the missiles and a MiG23 accompanying it, were from bases on Sakhalin, according to informed sources.

There has been no information released in Washington about other Soviet fighters that arose earlier from bases in Kamchatka when the airliner crossed over that peninsula, which houses sensitive Soviet military bases. Thus, there is no indication that the Soviets had any fighters near the airliner before the two that shot it down at Sakhalin.

A Soviet statement on the incident yesterday suggests that the fighters from Kamchatka may have been sent to investigate an American RC135 reconnaissance plane that the administration has acknowledged was also operating in the vicinity of Kamchatka on the same night, but outside of Soviet airspace.

The Soviet statement said the RC135, a four-engine military version of the 707 jetliner, was operating "near" the Soviet border, which seems to be a confirmation that the U.S. craft had not penetrated Soviet borders.

American officials will not say if they have any information about the Soviet fighters launched from Kamchatka. But at this point it appears that in the entire 2- to 2 1/2-hour period when Soviet ground radars were tracking the commercial jet, it was only in the last 20 minutes that Soviet pilots had the opportunity for precise identification in their own airspace. They apparently chose not to do so.

American officials yesterday also sought to demolish several Soviet explanations of activities. For example, the Soviets claim that, contrary to American intelligence reports, their planes are equipped with international emergency frequencies and that they tried to contact the airliner by radio.

U.S. officials continue to believe the Soviets do not have those frequencies in their fighters and that if the Soviets had made such a broadcast it would have been heard by the civilian control tower in Japan to which the airliner was routinely broadcasting position reports.

Similarly, the Soviets claim to have fired warning shots and tracers at the plane. But this is not supported by the transcripts. The Su15 that fired the heat-seeking missile that shot down the plane does not carry guns, according to the authoritative Jane's All the World's Aircraft. The MiG23 does carry guns but there is no record in the transcript that shows that plane fired at all.

Similarly, there is no indication from the commercial pilot in his reports to Japanese air controllers that anything was amiss during the flight such as tracer bullets being fired at him. Those normal radio transmissions continued until three minutes before the plane was shot down, according to the reconstruction of events as published here yesterday.

Those transcripts are based on recordings of the fighter pilots' communications sent to their ground commanders that were made by Japanese electronic listening posts.