A missile fired by a Soviet jet fighter downed a Korean Air Lines jet with 269 persons aboard near the Soviet island of Sakhalin Wednesday, American officials charged yesterday, calling on Moscow to explain "this appalling act."

The Boeing 747, whose passengers included Rep. Larry McDonald (D-Ga.) among an estimated 30 or more Americans, apparently plunged into the Sea of Japan, with no sign of any survivors. The plane went down early Thursday morning, Tokyo time, which was Wednesday afternoon here.

Soviet officials, facing mounting international outrage, did not acknowledge downing the plane. A brief statement issued in Moscow by the official Tass news agency said only that Soviet fighter planes had tried to guide "an unidentified plane" that twice "violated the air space of the U.S.S.R.," but that "the intruder plane did not react to the signals and warnings from the Soviet fighters and continued its flight in the direction of the Sea of Japan."

The Soviets later reported, in a message delivered to the State Department, that search parties had found signs of a possible crash in the area of Moneron Island, west of Sakhalin.

The plane, on a flight from New York to Seoul with a refueling stop in Anchorage, Alaska, apparently wandered far north of its scheduled flight path, which would have passed south of the Kuril Islands and the northern Japanese island of Hokkaido.

Secretary of State George P. Shultz said the plane "strayed into Soviet airspace over the Kamchatka Peninsula and over the Sea of Okhotsk and over the Sakhalin Island." Shultz said that "at least eight fighters" were scrambled to intercept the airliner, and that the fighter that shot it down "was close enough for a visual inspection of the aircraft."

But Shultz said there was no direct radio contact between the airliner and the Soviet planes. "We can see no explanation whatever for shooting down an unarmed commercial airliner," Shultz said.

In Santa Barbara, Calif., White House spokesman Larry Speakes announced that President Reagan was returning to Washington today, a day earlier than planned, for a meeting with his national security advisers.

Late yesterday, State Department spokesman John Hughes said Soviet Foreign Minister Andrei Gromyko had sent Shultz a message that was "almost verbatim" the Tass statement. Hughes said the Soviet Embassy was told that Gromyko's message was "totally inadequate" as an explanation.

Other nations with citizens aboard the downed plane expressed outrage. According to Korean Air Lines, the majority of the passengers were South Koreans, but there were also Japanese, Taiwanese, Filipinos, Thais, Canadians and persons of other nationalities aboard.

In Tokyo, Japanese Foreign Minister Shintaro Abe said that if the airliner had been shot down by Soviet planes, it was "very regrettable." Soviet Ambassador Vladimir Pavlov was summoned to the Foreign Ministry for an explanation.

In Ottawa, Canadian Minister of State Jean-Luc Pepin said that Soviet Embassy Charge d'Affaires Alexander Novikov cautioned him, "Planes go down without being shot down." Last night, Deputy Prime Minister Allan J. MacEachen said he was "very offended" by the Soviet reaction.

In Seoul, South Korean President Chun Doo Hwan met twice with his Cabinet during the day, and later issued a statement accusing the Soviet Union of "a barbarous act" and demanding a Soviet apology. South Korea has no diplomatic relations with the Soviet Union--a strong supporter of its rival, North Korea.

France also "expressed its gravest indignation" and instructed its foreign minister "to demand all the necessary explanations in order to determine who is responsible for this tragic affair."

The State Department said the United States was calling for a United Nations Security Council meeting on the incident.

Although the plane, Korean Air Lines' Flight 007, disappeared from radar screens at 3:38 a.m. Thursday, Tokyo time (2:38 p.m. EDT Wednesday), its fate was unknown for hours. When the plane failed to reach Seoul, Korean Air Lines announced that it had landed safely on Sakhalin. Yesterday, airline officials said they had made the announcement, based on "premature" information, to ease the concern of the passengers' relatives.

The airline's false announcement was just one of the mysteries surrounding the fate of Flight 007. The radar screens on which the flight was being tracked were operated by Japanese military personnel, who must have noticed that the plane had entered Soviet airspace. But civilian air controllers in Tokyo received radio messages from Flight 007 reporting, on the basis of its onboard instruments, that it was on course well south of Soviet territory.

Asked about this discrepancy, Japanese defense officials in Tokyo said they are not able to warn air controllers each time an unidentified plane enters Soviet airspace. Since the civilian controllers in radio contact with Flight 007 apparently did not have the plane on their own radar screens, they had no way of knowing the plane was not where its pilot had said it was.

Under internationally recognized procedures, aircraft intercepting a possible intruder wave their wings to indicate the intruder is to allow himself to be escorted to a landing.

It is unknown whether the Soviet fighters made such signals to the Korean jumbo jet. Asked last night on "Nightline" (ABC-WJLA) to speculate on why the Korean pilot may not have responded correctly, assuming the Soviets did signal, former deputy CIA director Bobby Inman said, "Most of those signals are visual." At night, he added, "it is very difficult." Inman said the Korean pilot "apparently didn't know" the plane was off course and perhaps "didn't understand the nature" of the Soviet signals.

Former CIA director Stansfield Turner, on the same program, said possible reasons why the pilot may not have responded "are difficult to divine." Turner said that during a similar incident involving a Korean Air Lines plane in 1978, "the same thing happened," and when the Soviets gave the signals, "the pilot didn't respond" until the Soviets fired a shot, damaging the plane and forcing it to land.

Turner, saying he was not apologizing for the Soviets, pointed out that the normally high level of Soviet "paranoia" could have been compounded "when the same country, the same airline does the same kind of thing."

When the plane was five hours overdue, and before the mistaken announcement from Seoul, the first assumption here was that it had crashed. At about 9 p.m. EDT Wednesday, however, South Korean Ambassador Lew Byong Hion contacted U.S. officials to express concern that the plane might have strayed over Soviet territory. At 10 or 10:30, Assistant Secretary of State Richard Burt said yesterday, the State Department was alerted that the plane had gone off course, and about an hour later, Burt said, a "working group" was formed.

By then, Burt said, reports were coming in that made it "increasingly apparent the plane had been forced down." At about midnight, Oleg M. Sokolov, the charge d'affaires at the Soviet Embassy here, was awakened and told that "we insisted on an explanation." Sokolov "had no information to offer," Burt said.

In Moscow, U.S. Embassy charge Warren Zimmerman called for an explanation from the Soviet Foreign Ministry, which disclaimed knowledge of the plane's downing, Burt said.

There was no authoritative explanation of why the airliner had been shot down, but the tragedy recalled a similar incident on April 20, 1978, when a Korean Air Lines 707 en route from Anchorage to Seoul was fired on and forced to land south of Murmansk by Soviet fighters, killing two passengers and injuring 13. Soviet officials said then that the 707 had violated their airspace and refused orders to land.

Both Soviet areas overflown in this case bristle with Soviet military bases. Kamchatka, one former top official of the Defense Intelligence Agency said yesterday, is "one of the most sensitive military areas in the Soviet Union." The peninsula, whose southern tip points toward the Kuril island chain, is used as a landing area for intercontinental missile tests and houses the home port for the Soviets' Pacific fleet of missile-carrying submarines.

Sakhalin, directly north of Japan's northern island of Hokkaido, has two Soviet air bases, two Red Army divisions and seven Soviet Navy facilities, according to U.S. sources. Both MiG23 fighters, initially identified by the Pentagon as involved in the incident, and older SU15 fighters are based on Sakhalin. U.S. defense officials said late yesterday that an SU15 fired the missile that downed Flight 007, but the White House last night said it was a MiG23.

Shultz refused to speculate yesterday on whether the attack on the plane had been ordered at a high level or by a local commander, but Assistant Secretary Burt emphasized that "the Soviet Union exercises very tight command and control over its combat forces, including fighter aircraft."

"There was continuous contact between the aircraft and ground control," Burt said. The pilot of the Soviet fighter plane, "communicating with ground control, described and discussed a sequence of movements and actions that he was taking to engage this aircraft, including the arming and firing of a missile."

Shultz also said that "the Soviet plane was, we know, in constant contact with its ground control." He emphasized that the airliner was tracked by Soviet radar "for some 2 1/2 hours," apparently suggesting this was time enough for consultation with officials in Moscow or the other nations involved.

Reports from Tokyo described in detail the last radio contacts between Flight 007 and the control tower at Tokyo's Narita Airport. The tapes of these contacts show that the Korean Air Lines pilot believed he was on course.

About an hour and a half before his plane went down, the pilot radioed the Narita tower, "We safely passed south of Kamchatka." This erroneous report apparently was based on a malfunction or misreading of navigational instruments.

An hour of radio silence followed, after which the pilot asked permission to increase his altitude from 32,000 feet to 35,000. The last clear message from Flight 007, at 3:23 a.m. Tokyo time (2:23 p.m. EDT), reported reaching 35,000 feet. The Tokyo air controllers said there was no sign of stress in the pilot's voice. Four minutes later the Narita tower heard some noise from the plane, but could not understand a word.

The initial Korean Air Lines report that the plane landed safely in Sakhalin relieved hours of anxiety for families of about 70 Koreans who had gathered at Seoul's Kimpo International Airport. Many of them returned home in relief. Then came news reports, late in the evening Korean time, that the plane had not landed.

Television stations in Japan began to show the weeping faces of relatives who had gone back to the airport.

It was not until about 7:30 last night, Korean time, that the government announced, to what was now a group of about 300 relatives gathered at the airport, its belief that the plane had been shot down.