The Reagan administration said for the first time yesterday that Korean Air Lines Flight 007 may have been in international rather than Soviet airspace when it was shot down by a Soviet jet fighter.

This suggestion is contained in an official U.S. statement issued late yesterday, which said, in part, that "the Soviets shot down the Korean airliner as it was exiting (or had exited) their territory west of Sakhalin Island . . . ."

Administration officials declined to reveal last night what evidence or analytical information they had to support this suggestion. One official said only that the statement was "carefully worded and we have made scrupulous efforts to be careful."

Until now, administration officials have said that the Boeing 747 passenger jet with 269 aboard was shot down, in the words of Richard Burt, an assistant secretary of state, "as it was leaving Soviet airspace."

If the plane had in fact left Soviet airspace by the time it was hit, the incident could prompt even more world condemnation. The administration already has stressed that the tragedy is international in scope and requires some international response. According to accounts of the episode released last week by the State Department and based on monitored Japanese recordings of reports from Soviet fighter pilots to their ground commanders, the Korean Air Lines plane crashed into the ocean about 12 minutes after the pilot of a Soviet Su15 fighter reported that he had fired a missile at "the target" and that it was destroyed.

This means the plane may have glided some distance before it hit the water. Most of the wreckage of the plane is believed to have fallen in Soviet waters, which extend 12 miles from Soviet territory. Repeated requests from the United States and Japan for permission to join in a search operation in those waters have been refused by the Soviets.

The U.S. statement containing the suggestion that the Korean Air Lines plane may have been hit after it left Soviet airspace also contains the claim that the Soviets "made no serious effort to identify the aircraft or to warn it. They did not appear to care what it was. Instead, they were intent on killing it."

That is the furthest the Reagan administration has gone in suggesting that the Soviets did not necessarily know precisely what they shot at. Previously, U.S. officials have stated that it would have been virtually impossible for the Soviet fighter pilots, even at night, not to recognize the plane as a Boeing 747 passenger airliner with its large size and distinctive shape.

Secretary of State George P. Shultz, in his initial account of the incident Friday, reported that Soviet jet fighters had made "visual contact" with the airliner.

President Reagan said in his nationally televised speech last night that the monitored recordings of the reports of the Soviets pilots to their ground commanders indicated they came to "within about a mile of the Korean plane," that it was "a clear night with a half moon" and that they saw the plane's navigation lights were on and its strobe light blinking.

"There is no way a pilot could mistake this 747 for anything other than a civilian airliner," Reagan said.

Yet, the U.S. statement issued in conjunction with the president's speech said the Soviets "made no serious effort to identify the aircraft . . . ."

Interviews with pilots yesterday produced differing views. Some said it is possible to distinguish a 747 from a mile or two away at night with good visibility. Others disagreed, saying little can be seen clearly enough at that distance. All agreed that nighttime identification of an airliner by company or country required observation from close, perhaps 100 yards or less.

At no time on the monitored communications did the Soviet fighter pilots refer to the airliner as anything but "the target."

The question of whether the Soviets knew what they shot at was made more difficult to answer this weekend when it was revealed that a U.S. Air Force RC135 reconnaissance jet was flying at one point in the vicinity of the Korean Air Lines plane when both were outside Soviet airspace. The RC135 is a four-engine, military version of the 707 jetliner and about half the size of a 747.

In Reagan's speech last night and in the government statement issued before it, the administration went to great lengths to explain why the Soviets should never have mistaken the South Korean airliner, which eventually strayed more than a hundred miles into Soviet airspace, from the U.S. reconnaissance plane, which stayed in international airspace and was 1,000 miles away, back at its base in Alaska, when the airliner was shot down.

The U.S. statement said, however, that "as the Korean airliner strayed off course and overflew the Kamchatka Peninsula which is Soviet territory and a missile test area it was initially identified by the Soviets as an RC135 and then as an unidentifed aircraft."

This would mean that at least in the initial phase of the incident, the Soviets on the ground thought the intruding airliner was a U.S. spy plane and that their pilots went aloft with that in mind.

Because of this and the apparent lack of evidence to suggest that the Soviet fighter pilots ever identified the plane, it remains unknown whether the Soviets intentionally shot down a commercial airliner or were guilty of what several officials have said could have been an almost unimaginable mistake or extreme callousness.

The U.S. statement said the Soviets are well aware of the U.S. reconnaissance flights and track them routinely on radar. "They know that our aircraft do not enter their airspace," the statement said, and the Soviets also knew there were two aircraft on their ground radar screens at the time.

Thus, the statement said, the fact that one strayed into Soviet territory "should" have been an indication to the Soviets that it was not the reconnaissance jet.

On a radar screen, large jet aircraft of different types show up as the same kind of electronic blip.

The U.S. statement said the Soviets tracked the two planes separately and "knew" there were two aircraft in the area, so we do not "think" this was a "a case of mistaken identity."

The U.S. reconnaissance plane, which is used to monitor Soviet missile testing, was on a circular flight path that came within 75 miles of the South Korean airliner, according to the statement. The Soviets shot down the airliner 2 1/2 hours after it was detected intruding into Soviet airspace, by which time the U.S. reconnaissance plane was 1,000 miles away, according to the U.S. account.

In reporting the "incontrovertible evidence we have" of the Soviet action, Reagan said that, while "no one will ever know" how the Korean Air Lines pilot got so far off course, the Soviets tracked it for 2 1/2 hours as the plane flew a straight line at 30,000 to 35,000 feet.

While over the Pacific in international waters, the statement said, Korean Air Lines Flight 007 "was for a brief time in the vicinity of one of our rceonnaissance planes, an RC135, on a routine mission. At no time was the RC135 in Soviet airspace. The Korean airliner flew on and the two planes were soon widely separated."

Reagan said the Soviets eventually scrambled fighters from bases on Sakhalin Island, as Japanese ground stations recorded the transmissions of the pilots to their military commanders on the ground. The transmissions from the ground commanders to the fighter pilots "were not recorded," however.

This would explain why there is no recorded Soviet order to fire the missile, but Reagan said "it is plain, however, from the pilot's words that he is responding to orders and queries from his own ground control."

The president reported that the Soviet pilot described his search for what is only called "the target." The pilot saw a target with navigation lights on, Reagan said, and "reports he is reducing speed to get behind the airliner." The word airliner was used by the president, not by the Soviet fighter pilot.

Such a maneuver would be common for a pilot preparing to fire a missile, to give himself some distance from the slower moving target.

The Soviet pilot then told the ground commanders that he has "locked" his radar on the target, "which aims his missiles, has launched those missiles, the target has been destroyed, and he is breaking off the attack."

The Soviets apparently fired two missiles rather than just one. In addition to the Su15, a MiG23 fighter also was reported to be flying nearby and his voice also is on the recordings.