White House national security affairs adviser Zbigniew Brzezinski cautioned yesterday against any hasty U.S. response to a Soviet plan to reduce the number of tanks and troops in East Germany and warned that the withdrawals could be designed to upset Western defense decisions.

"We have to study this very carefully before responding in a final conclusive fashion," Brzezinski said during a taped appearance on "Issues and Answers" (ABC, WJLA).

At ceremonies marking the 30th anniversary of the founding of East Germany on Saturday, Soviet President Leonid Brezhnev said the Russians would unilaterally remove up to 20,000 troops and 1,000 tanks from East Germany over the next 12 months.

Brezhnev also offered to reduce the number of nuclear medium-range rockets aimed at Western Europe if the Atlantic alliance would hold off on the planned introduction of nuclear missiles of similar range in Western Europe.

"We welcomed the initiative in one respect," Brzezinski said yesterday. "But in another respect we have to recognize that the Soviet Union has an interest in forestalling Western efforts to upgrade our conventional and theater nuclear forces in order to achieve equality."

Brzezinski noted that Western plans to improve conventional weaponry in Europe, and a move toward a plan for achieving a medium-range nuclear missile response capability there, were in response to similar Soviet upgrading of Warsaw Pact conventional and nonconventional weaponry.

"There may be some elements in his speech which are designed to forestall the needed Western defense decisions," said the White House adviser.

NATO ministers are expected to vote approval in December for deployment of U.S.-built Pershing and cruise missiles in Western Europe, giving NATO capability for the first time of hitting targets inside the western perimeter of the Soviet Union.

Final approval, however, rests on parliamentary agreement in the nations slated to get the missiles.

Brzezinski said yesterday that plans for the missile deployment would continue in order to make up for what he called "growing asymmetry" in arms strength in favor of the Warsaw Pact over NATO.

"We do feel that we have to offset the existing inequality, an inequality which is threatening to the West and which could make for instability," said Brzezinski.