American-Soviet relations are "on the upswing" in what is bound to be a continuing mix of "cooperation and competition," presidential national security adviser Zbigniew Brzezinski said yesterday.

Nuclear arms control negotiations with the Soviet Union have raised the prospect "for the first time in 30 years" of "an actual numerical reversal in the strategic levels" of the arms race, he said.

In addition, Brzezsinki told an Overseas Writers Club luncheon, "we're imposing genuine restraint" on developing new weapons systems in the current U.S.-Soviet strategic arms limitation talks (SALT).

Critics are protesting that this will fall far short of the original SALT goals set out by the Carter administration last March. Brzezinski, acknowledging that, said that nevertheless there will be important gains over the nuclear negotiating terms that the Carter administration inherited.

In the Middle East, Brzezinski said, the U.S.-Soviet Oct. 1 statement on principles for recovening a Geneva conference on an Arab-Israeli peace settlement showed "some narrowing of U.S.-Soviet differences" and "also some moderation of Soviet policies or formulations regarding some very sensitive issues."

This does not mean, however, he said, that there is "a joint U.S.-Soviet concept of a settlement." He said, "We want the parties themselves to negotiate the settlement."

Pressed to say what he meant, as stated in a recent interview, by using American "leverage" to help induce the parties to achieve a settlement, Brzezinski said at one point: "The fact of the matter is that all military or economic arrangements between Israel and the United States are negotiated between the two parties. That is a fact of life . . ."

But, he said, "I think a threat, direct or indirect, to the economic or political or security viability of Israel would have the most devastating political and psychological consequences."

No "morally sensitive person," Brzezinski said, can "forget what has happened in the last 30 or 40 years in the world," alluding to the mass killings of Jews in Europe during World War II.

In reviewing U.S.-Soviet relations, Brzezinski said the American people must learn to live with a process of competition and cooperation, and to avoid the "extremes of cuphoria or pessimism" as events shift from one trend to the other.

At present, he said, "the cooperative aspects are indeed on the upswing, but this should not be interpreted as meaning that the competition is being terminated."

His accounting of U.S.-Soviet nuclear negotiations avoided all specifics in the still-secret negotiations, although the terms have been unofficially publicized.

The arms ceilings of 2,400 intercontinental nuclear missiles and bombers on each side, set at Vladivostok in 1974, Brzezinski said, "created an illusory equality," because "we had absolutely no intention to reach" that level.

Now, he said, there is hope for "a step down" from the Vladivostok ceilings; some real arms cuts; "constraint" on the "modernization of certain strategic systems," and "confidence-building" limitations on Soviet and American weaponry to head into "more ambitious objectives" in later negotiations.