Presidential assistant Zbigniew Brzezinski, opening a new phase of debate on the nearly completed SALT II treaty, described its provisions last night as vital to establish an agreed strategic framework with the "next generation of Soviet leaders."

Brzezinski's address to the Chicago Council of Foreign Relations was described by White House Officials as the political half of a two-part opening statement by the Carter administration in an accelerating national debate on the strategic arms limitation treaty (SALT).

Secretary of Defense Harold Brown is to provide the accompanying security justifications in an dress today to the Council on Foreign Relations and Foreign Policy Association in New York City.

When the Brzezinski and Brown addresses were scheduled a few weeks ago officials acknowledged, it was expected that negotiations with the Soviets on the main provisions of the treaty would be completed by now. The decision to go ahead with the speeches reflects a belief that completion may be only days away, depending on Moscow's reply to U.S. proposals late last week on final outstandin issues.

Brzezinski's speech addressed, for the first time in a major forum, the political problem of conluding a far-reaching treaty running through 1985 with a Soviet government headed by a leader in failing health. The national security adviser argued that the expected change of the Soviet leadership "during the next several years" makes it important to put in place strategic nuclear limitations today.

"When the next generation of Soviet leaders decides its policies toward strategic arms and toward the United States, we want them to face clear and agreed restraints on the competition in strategic arsm," Brzezinski said in a text released by the White House.

He added that "a vital objective" of SALT II is "helping to determine the way in which the Soviet Union will see future relations with the United States."

In anothe reflection of growing concern about the health of Soviet President Leonid I. Brezhnev, Carter administration officials made it known for the first time that the president will consider going to Europe for the summit meeting to sign the SALT agreement if BREZHNEV IS UNABLE TO COME HERE.

UNTIL THIS WEEK, THE WHITE HOUSE AND STATE DEPARTMENT HAD INSISTED THAT THE SUMMIT SHOULD BE IN THE UNITED STATES BECAUSE THE LAST MEETING OF THE TWO NATIONS' LEADERS TOOK PLACE IN THE SOVIET UNION.

THE SHIFT IN THE U.S. position is attributed by officials to "cumulative evidence" that the 72-year-old Soviet leader is in failing health. Last week the Soviets abruptly postponed a meeting with French President Valery Giscard d'Estaing for a month, saying that Brezhnev is ill with flu.

In view of the known schedule of Soviet leadership meetings, the Giscard visit now scheduled for April 26-28 and the May Day observance in Moscow, the summit meeting is not expected before mid-May Administration officials made it clear that Carter does not expect to go to Moscow, since such a gesture would be seen by some as demeaning.

But they are now willing to contemplate Geneva or some alternate European site if the Soviets cite Brezhnev's health as the reason for restricting his travels.

Brzezinski's address, expressing allout support for SALT II, including his insistence that strategic arms limitations cannot be linked to Soviet actions in other fields, is likely to quiet speculation that the national security affairs adviser will be only lukewarm in support of the expected treaty.

Brzezinski, a leading figure in a controversy over 'linkage' of SALT to Soviet actions in Africa a year ago, argues last night that Washington would "stronglky reject" any Soviet effort to link the strategic arms limits to Chinese-American relations.

"By the same token we cannot insist that the Soviet Union accommodate us" in matters of U.S. concern as a condition for strategic arms agreements. Imposing such linkage would be "not a prescirption for policy but for paralysis," Brzezinski said.

Concerning the controversial and complex problems of verification, Brzezinski said the United States is Brzezinski said the United States is "fully confident" that the treaty can be adequatley verified. He described the U.S. means of verification as "totally under our own control," and siad that, while those means cannot be "absolutely perfect," the administration is confident of detecting "any significant violation" before it would have to react militarily.

In a related development, administration officials confirmed a New York Times report of plans to use a modified version of the U2 high-altitude spy plane to monitor Soviet missile tests, replacing now-closed electronic listening posts in Iran. The U2s would be flown close to the border but not over Soviet territory, the newspaper said.

Senate Assistant Minorit Leader Ted Stevens (R-Alaska) told the Senate that information about the U2 plan was relayed to key senators on a highly classified basis Tuesday. He accused the Carter administration of "turing into a sieve" and of using one set of security classifications for the Senate and another for the press.

Administration officials appeared to be startled by the Times account, by Washington correspondent Richard Burt. Officials said the decision on the U2 flights was made Tuesday in an anannounced meeting of a top-level National Security Council committee, and that Burt was asking questions before the meeting was even concluded.