LONDON, SEPT. 17 -- NATO Secretary General Lord Carrington today welcomed an expected U.S.-Soviet agreement on eliminating medium-range nuclear weapons, but warned that the Atlantic Alliance "should not let any euphoria it engenders push us toward some mythical non-nuclear Nirvana."

Reflecting the concerns of many in Western Europe, Carrington cautioned against any weakening of NATO's flexible response strategy, saying that "nuclear weapons will remain essential" to alliance defense.

Nevertheless, he said, an agreement on intermediate nuclear forces (INF) "will change the landscape of Europe . . . perhaps as profoundly as any development in a generation." Its implementation, he said, would require NATO members to "look long and hard" at how arms control fits into overall alliance strategy, and prepare to make some "difficult decisions" on priorities and funding.

Carrington's remarks, in a speech to the Royal Institute of International Affairs here, came as U.S. Secretary of State George P. Shultz and his Soviet counterpart, Eduard Shevardnadze, were holding their final meeting on the proposed accord in Washington.

Carrington said he did not expect the agreement to lead to what some fear will be a "slippery slope" of divisions between the United States and Europe. The "tangible manifestation of American commitment" to Europe was not "in-theater missiles," Carrington said, but the presence of 326,000 American troops.

NATO's basic unity, he said, was ensured by the fact that the United States believed the continued presence of its troops in Europe was essential to America's defense.

But Europe, he said, must face the "bittersweet fact of life" that the future of European security "will come from decisions taken in Washington and in Moscow."

Carrington described an INF accord as "an historic achievement which we should welcome. For the first time in arms-negotiating history there is the prospect of phasing out a whole category of weapons." The agreement "could also bring an immense improvement in East-West relations and, I hope, presage further progress in the arms control field" and movement on a chemical weapons ban and reductions in conventional forces.

Meeting the new challenges of a post-INF world "will, in effect, be NATO's agenda for at least the next five years and probably for some time after that," he said.

Immediately after an accord takes effect, he said, "we shall confront the need to maintain stability during treaty implementation." He warned that "temporary vulnerability and perhaps imbalances might arise and could pose risks which we must guard against" during an implementation period of three to five years.

Speaking of prospects over the longer term, Carrington echoed the concern expressed earlier this week by NATO's new supreme commander in Europe, Gen. John Galvin, that the alliance not pull back from its commitment to modernize its European tactical nuclear weapons stocks, as a way of preserving deterrence after the mid-range weapons have gone.

NATO military commanders had also stressed the need to retain other systems, particularly aircraft capable of carrying both conventional and nuclear weapons, Carrington said. Effective deterrence "does not require a magical number of nuclear weapons. But it will always require a sufficient number and mix of systems, albeit at lower levels than before."

He cautioned against attempts to proceed quickly after an accord toward agreements on reductions in other nuclear weapons categories. Carrington referred to an article in today's Soviet Communist Party paper, Pravda, in which party leader Mikhail Gorbachev said it might be possible to conclude a pact cutting superpower strategic nuclear arsenals in half by the end of next year.

Carrington noted that Gorbachev had "given no indication that he has removed" Moscow's demand for restrictions on the U.S. Strategic Defense Initiative, popularly known as "Star Wars," before strategic arms reductions talks can go ahead.

Changes in Soviet attitudes and policy under Gorbachev had led to progress that was unthinkable when Carrington began his term as secretary general in 1984, he said. But there was "room for skepticism" about Gorbachev's goals, and "forecasts of Soviet intentions run two ways: toward greater detente on the one hand, towards seducing us, siren-like, into dropping our guard on the other."

Arms control "can and should be only one aspect of a wider detente process," Carrington said. "But it could also be a test of our own resolve; not to be carried away in a wave of euphoria at the first sign of an arms agreement."