Secretary of State George P. Shultz today denounced Soviet Foreign Minister Andrei Gromyko's "totally unacceptable" response in face-to-face discussions on the Korean Air Lines disaster, plunging Soviet-American diplomatic relations to a new low.

Shultz's strong words followed a two-hour meeting with Gromyko--the first top-level diplomatic meeting of the two countries in almost a year--in which the U.S. diplomat sought a detailed explanation of the Soviet attack on the Boeing 747 with 269 persons aboard and assurances that such an incident will not happen again.

Gromyko's response in private, Shultz told cameras and reporters, was "even more unsatisfactory" than the one he made in a speech here Wednesday. "I find it totally unacceptable," said Shultz.

Gromyko declined comment as he left the session in the U.S. Ambassador's residence.

An aide to Shultz said later that Gromyko's statements today did not differ in substance but that Gromyko's tone was "more aggressive" in the private meeting.

Shultz said the United States will continue its diplomatic efforts to press for remedial measures.

In a meeting attended by Shultz late tonight, NATO foreign ministers moved closer to adopting a package of measures in reaction to the downing and scheduled a meeting of aides in Brussels Friday to work further on an accord.

A prominent feature of the package, U.S. sources said, is a governmentally endorsed suspension of commercial air traffic to and from the Soviet Union, probably for about two weeks beginning around Sept. 15.

Of the 16 NATO countries, only France and Greece were reported to have spoken negatively about the suspensions in the meeting tonight. The suspension was proposed by West German Foreign Minister Hans-Dietrich Genscher, who described it as "a political signal" to the Soviets rather than a sanction. Non-NATO countries will be asked to approve the suspension, which is likely to coincide with a work stoppage by pilots and possibly other airline personnel involved in Soviet-related flights.

Other features of the emerging package, the U.S. sources said, were:

* Support for the Korean government's five demands to the Soviet Union for a formal accounting of what happened, monetary compensation, punishment for those responsible, access to the crash site and guarantees against a future repetition.

* Proposals to the international civil aeronautics meeting next week in Montreal for changes in procedures and international rules to facilitate communication between civilian and military aircraft and air controllers and a flat ban on attacks against civilian airliners in peacetime.

Meanwhile, the airline boycott continued to spread as British Airways announced in London the suspension of all flights to the Soviet Union for 60 days, and Swiss and Italian pilots voted to join the boycott of the Soviet Union, according to news agency reports.

According to a U.S. participant in the meeting, the Soviet minister at first did not wish to discuss the airline disaster, but Shultz insisted.

The official, who declined to be quoted by name in a briefing for reporters, left the impression that Shultz would not proceed with the discussion until Gromyko agreed the airliner would be a topic.

Gromyko then turned to what the U.S. official called "a lengthy, somewhat stale presentation" of Soviet position on arms control. Shultz reportedly responded by describing U.S. positions in the negotiations with the Soviets on medium-range missiles in Europe, strategic arms and several other sets of talks.

Shultz asked several direct questions of Gromyko, the U.S. official said, and demanded an explanation of the downing and assurances against future repetitions.

The U.S. official said none of the questions were answered to Shultz's satisfaction, and that Gromyko's basic response was "to shift the responsibility to the United States." Gromyko charged yesterday that the U.S. was responsible for the disaster because, he claimed, the airliner was flying over strategic areas of the Soviet Union while on "special duty" for U.S. authorities.

Shultz rejected Gromyko's explanation when the Soviet minister advanced it before a meeting of the 35-nation European Security Conference yesterday. Following today's meeting, Shultz called the Soviet explanation "preposterous."

In Moscow, the Soviet news agency Tass said the Soviet side had concentrated on arms issues while the United States avoided any discussion of the "essence" of these problems, Washington Post correspondent Michael Dobbs reported. Tass said Gromyko accused the United States of worsening Soviet-American relations and of "a gross provocation" over the incident.

Although he responded to Gromyko's remarks on arms control, aides said, Shultz did not negotiate on the subject or explore areas for possible agreement as he had considered doing in the meeting here prior to the downing of the South Korean airliner a week ago.

Shultz's denunciation on the doorstep of a meeting with Gromyko had little precedent in the recent diplomatic relations of the two superpowers, and raised new doubt whether their dialogue will continue when Gromyko comes to New York later this month for his annual visit to the U.N. General Assembly.

From the start, the session here was different. It was U.S. Ambassador to the Soviet Union Arthur Hartman, not Shultz, who met Gromyko.

Inside the residence of U.S. Ambassador to Spain Thomas O. Enders, Shultz, Gromyko and their aides sat at a small oval dining table.. There were no glasses of water or soft drinks, no pads and no pencils on the table. Shultz originally had planned a luncheon as part of the meeting, but canceled the meal after the incident.

The meeting was about twice as long as Shultz planned, apparently due to the arms-control exchange. It was still a far cry from the marathon sessions that have often been held with Gromyko even in periods of stress, such as the 7 3/4-hour session between former secretary of state Alexander M. Haig and Gromyko in January 1982 within a month of the imposition of martial law in Poland.

Shultz ended today's meeting, the U.S. official said, when "it became clear that further discussion of the subject [of the airline disaster] was not going to be productive."