The Soviet News Agency Novosti said tonight that the deployment of new medium-range American nuclear missiles in Western Europe would force the Soviet Union to switch to an "instantaneous" retaliatory posture. The so-called "launch on warning" would prompt a Soviet counterstrike if monitoring equipment signals a U.S. attack.

The agency said the deployment of 572 Pershing II and cruise missiles, commonly known as Euromissiles in Western Europe, which is scheduled to begin at the end of 1983, would create a new strategic situation. Their proximity to Soviet borders "rules out the possibility of preventing a conflict, should one be started, by nonmilitary means."

Although it did not use the term "launch on warning," the Novosti article clearly referred to that concept as used by the Americans.

Today's article went further in specifying Moscow's response than any previous Soviet pronouncement on the subject. Western diplomats noted, however, that it coincided with the scheduled NATO defense ministers' meeting in Brussels this week and that it may be designed to scare West European and American public opinion.

U.S. Defense Secretary Caspar W. Weinberger arrived in Brussels Monday for a four-day NATO strategy conference, The Associated Press reported. The 12 West European defense ministers issued a communique at the start of the meeting aimed at quieting criticism in the United States that the Europeans are not contributing their share to the alliance and therefore some of the Americans stationed in Europe should be withdrawn.

[The communique said that although Europe has about half the alliance's population and gross national product, it provides about 75 percent of NATO's ground forces, combat aircraft, tanks, armored divisions and fighting ships in Europe.]

Novosti quoted Soviet military circles as contending that the NATO deployment "inevitably demand[s] from the Soviet Union instantaneous actions in reply.

"Faced with an infinitesimal warning time, the only possibility remaining is a nuclear retaliatory strike in retribution. There is no other alternative," it said. "The few minutes' flight of a Euromissile will undoubtedly become the first minutes of an all-European and world nuclear catastrophe."

The ostensibly semiofficial agency is used by the Kremlin to advance Soviet ideas without giving them the formal authority carried by the official news agency Tass or the Communist Party newspaper Pravda.

If actually implemented, the posture of launch on warning would not only increase the risk of a nuclear war but also would be highly destabilizing under the best of circumstances.

So far, all nuclear powers have operated on the basis of "launch on attack" -- or the political leadership reserving the right to order a retaliatory strike when it is convinced that its country is being attacked by an adversary.

"Launch on warning" would be a posture under which Soviet missiles would be programmed for instant action if computerized Soviet monitoring facilities reported an imminent American threat.

Novosti tonight referred to the fact that Pershing II, if stationed in West Germany, would hit Soviet targets within eight minutes of being launched. Cruise missiles, although slower, were said by Novosti to have the same capability of carrying out a surprise strike because they are hard to detect.

While not discussing possibilities of Soviet equipment failures or wrong alerts, Novosti said that the American medium-range nuclear weapons might be launched either deliberately or as a result of a "subjective human mistake" or a technical failure.

Moscow, it said, would have to take this into account and "a retaliatory strike would be carried out not only on American launch sites, but also on command points, communication centers and arms stockpiles, many of which are directly situated, as is well known, in densely populated areas of West European countries."

Marshal Dmitri Ustinov, the defense minister, indicated in July that a switch to launch on warning was being considered as a serious option to what is seen here as a shift in the strategic balance if the NATO deployment is implemented. But Ustinov writing in the Communist Party newspaper Pravda, was not explicit. He wrote that given "the current state of the systems of detection and the combat readiness of the Soviet Union's strategic nuclear means, the United States will not be able to deal a crippling blow to the socialist countries" and that Washington "will not be able to evade an all-crushing retaliatory strike."

The Soviet Union, he added, would defend itself "without any hesitation."

Political observers here believe that, apart from their propaganda aims, the Soviets would, in fact, adopt "launch on warning" as a preliminary and relatively cheap response to President Reagan's military policies.

Well-informed sources here say that Moscow continues to wait for some sort of concrete conciliatory gesture from President Reagan. According to this view, the new Kremlin leadership is continuing its reassessment of Soviet strategic needs and responses.

But the president's announcement on deploying the MX missile has clearly destroyed an optimistic mood generated by the visit here of Vice President Bush and Secretary of State George P. Shultz for the funeral of president Leonid Brezhnev.