President Carter yesterday rebuffed Soviet President Leonid Brezhnev's proposal for military reduction in Europe and urged full support for NATO plans to place longer-range nuclear missiles on the continent.

Carter's remarks in a news conference while falling short of a formal rejection, left no doubt of his negative attitude toward the proposal announced by Brezhnev last Saturady in East Berlin.

"It's not quite as constructive a proposal as at first blush it seems to be.

I think it's an effort designed to disarm the willingness or eagerness of our allies adequately to defend themselves," Carter declared.

The president went on to urge a western alliance decision to modernize its European military strength and then to negotaiate with the Soviets to lower armaments on both sides.

A "high-level group" of North Atlantic Treaty organization officials has recommended approval of U.S. plans to station new 1,000-mile-range Pershing 2 missiles and 1,500-mile-range ground-launched cruise missiles in Western Europe to replace the 400-mile-range Pershing 1. The plan, in response to Soviet deployment of 2,500-mile-range SS20s that threaten Europe, is scheduled to be considered by NATO defense ministers in mid-December.

Brezhnev, on the other hand, called for a halt to the NATO plan, which he said would "radically alter the strategic situation on the European continent [and] poison the international atmosphere." The Kremlin leader offered a future reduction in his nations's medium-range missiles targeted on Europe in return for a NATO freeze, but indicated that Soviet missile strength will grow if NATO moves ahead.

The Brezhnev proposals, including a unilateral offer to withdraw up to 20,000 Soviet troops and 1,000 Soviet tanks from East Germany in the next 12 months, have been given full-volume treatment by Soviet media as a "historic peace initiative."

Carter, in his response to the Brezhnev plan, said the Russians have been replacing their older airplanes with new Backfire bombers and their older SS4 and SS5 missiles with new SS20 weapons carrying three warheads each and packing much greater range and accuracy. The new Soviet missiles are also mobile and thus less vulnerable to an allied "preemptive strike," he said.

Carter said Brezhnev is offering, in effect, to continue the Soviet rate of modernization "provided we don't modernize at all."

At another point, however, Carter called Brezhnev's plan "an interesting proposal . . . that might show promise" but he immediately added that "it is not as great a step as would ordinarily be judged at first."

Carter took the occasion to call anew for Senate approval of the strategic arms limitation treaty (SALT II) with the Russians, which limits strategic armaments but does not apply to intermediate-range missiles. He noted that future negotiations on Eastern and Western European military cuts, which are expected to take place in a SALT III forum, are dependent on approval of the strategic pact now before the Senate. For this reason among others, Western European leaders have made strong statements of support for SALT II.

The latest Carter-Brezhnev public exchange came in the wake of unsuccessful efforts to achieve a negotiated settlement of the dispute over U.S. charges of a Soviet "combat brigade" in Cuba.

"In my opinion we have answered the question of the Soviet combat unit in Cuba adequately. I think we have isolated any treat from that unit. We will increase our surveillance there. I believe it has been adressed adequately," Carter said in response to a question.

While calling the presence of a Soviet unit in Cuba "a serious matter," Carter said it is "not the most important matter" and not "a major threat." He said the countermeasures he announced last week were "measured and appropriate."

Responding to former secretary of state Henry A. Kissinger's sharp criticism of the handling of the Soviet-troops-in-Cuba issue, Carter said Kissinger's basic premises of concern were "compatible" with his and that he had seen no report that Kissinger had recommended different moves than had been adopted.

At the State Department earlier in the day, spokesman Hodding Carter delivered a more combative response. Kissinger is "wrong" and will change his mind about the gravity of the threat that the Soviet force poses, the spokesman said.

The president, as he often has in the past, delivered a bitting attack on the Soviet Union as a totalitarian, atheistic and military-oriented nation whose values and accomplishments compare poorly with those of the United States in political, religlous and economic fields.

At the same time, he credited Brezhnev with sharing a common interest in avoiding a nuclear war.

In response to a question about use of the planned "rapid deployment force" that was given emphasis in his address on the Soviet troops, Carter said he sees "no prospect at this point for our intervention militarily any place in the world."

He also said, responding to a query about the impending famine in Cambodia, that the United States is "ready and eager" to join with other nations to provide humanitarian aid.

Carter said he does not plan to meet with the Rev. Jesse Jackson about his just-completed tour of the Middle East, which ended with a recommendation for U.S. official dealings with the Palestine Liberation Organization. There is no change in the U.S. posture on the PLO, said Carter.