National security affairs adviser Robert C. McFarlane said yesterday that the Soviet proposal for a 50 percent cut in nuclear warheads would increase Moscow's ability to launch a "first strike" against the United States but that the plan also offers a "constructive beginning" for superpower negotiations.

Appearing on NBC's "Meet the Press," McFarlane appeared to be trying to steer a middle course between those welcoming the Soviet proposal and those saying it would put the United States at a disadvantage. Both points of view are represented in the Reagan administration.

Presenting U.S. objections to the plan put forward by Soviet negotiators in strategic arms talks at Geneva last week, McFarlane said Soviet leader Mikhail Gorbachev "is telling us, if you will get rid of your defense, I will get rid of your offense."

Part of the Soviet proposal calls for "cessation of work" on the Strategic Defense Initiative, President Reagan's proposal for a missile defense. McFarlane defended it yesterday in general terms without saying what, if any, limits the president would accept on testing and development of the system.

Asked if he thought that the Soviets would insist that the United States accept at least some limitations on SDI as a condition for proceeding with arms-reduction negotiations, McFarlane said, "I don't think so."

Reagan, in a speech Friday, reiterated his commitment to "research and testing" of SDI.

On the Mideast peace issue, McFarlane said "some milestones of progress" could be achieved "within a month's time" but declined to be specific. U.S. officials said last week that they were making some progress on obtaining international auspices for peace talks involving Jordan, Israel and Palestinan representatives.

Israel Prime Minister Shimon Peres, who is to visit here Oct. 16, said last week that Israel would be willing to accept involvement of permanent members of the U.N. Security Council with whom it has diplomatic relations -- the United States, Britain and France.

On Saturday night, according to diplomatic sources, Secretary of State George P. Shultz telephoned Israeli Foreign Minister Yitzak Shamir to assure him that U.S. policy toward Israel or terrorism has not changed despite adoption of a Security Council resolution condemming Israel's raid against the Palestine Liberation Organization in Tunisia last week in which Tunisians were killed.

On SDI, McFarlane said in yesterday's interview that Reagan intends to pursue SDI "through a vigorous research testing program, which is clearly consistent with the 1972 ABM Treaty."

McFarlane asserted that research, testing and development of new defensive weapons "involving new physical concepts," as much of the SDI technology does, "are approved and authorized by the treaty. Only deployment is foreclosed," he added.

The ABM Treaty, which outlaws space-based missile-defense systems, includes an "agreed statement" that is its only reference to exotic new weapons based on "other physical principles." The statement says that "to ensure fulfillment of the obligation not to deploy ABM systems" in space, in the air or at sea, the two superpowers agreed that any new type of antimissile system "would be subject to discussion . . . and agreement" in accordance with the provision of the treaty explaining how it could be amended.

Asked yesterday about McFarlane's claim that the ABM Treaty approves and authorizes SDI testing and development, Gerard C. Smith, the Nixon administration official who negotiated it, said, "He's got it all screwed up."

Smith, director of the Arms Control and Disarmament Agency in the first Nixon administration, said the treaty "just doesn't" allow testing on space-based systems. "All that is permitted for space-based systems is research," Smith said. "That means in a laboratory."

Informed of Smith's comments, McFarlane said, "It's a simple matter of reading the text. The fact of the matter is that when you are dealing with new principles -- and by that we mean principles other than ground-to-air missiles -- research other than pure research in the laboratory is allowed."

Detailing U.S. objections to the Soviet proposal, McFarlane said, ". . . The proposal is not 50 percent of offensive, strategic missiles.

"It is 50 percent, counting U.S. systems, of not only strategic, but of medium-range systems, of aircraft . . . and of systems that don't have any relevance to the strategic equation. In addition, it excludes on the Soviet side . . . over 1,300 warheads on SS20s that threaten our allies.

"In so many words, it says that it's all right for the Soviet Union to threaten Europe, but it isn't all right for Europe to defend itself," McFarlane said. "In a nut shell, it requires the United States to choose whether we will defend our allies or we will defend ourselves, for in the limits they propose you couldn't do both."

Later in the program, McFarlane took a more positive tone. For years, the United States has urged the Soviets to agree to limits on offensive missiles. McFarlane was asked why he did not "declare a victory" over the new proposal. He clapped his hands and said, "I do applaud the commitment to reductions, and I don't mean to be frivolous about that. There are elements in that proposal that we find a very constructive beginning but not an end . . . I don't mean to be negative on the progress which has now begun in earnest, and the president's very committed to getting as much out of it as he can."

However, McFarlane said he doubts "that even the Soviet Union pretends" that its proposal is "an equitable basis for an outcome." If the United States accepted the plan, he said, the Soviets could target six warheads on each hardened silo containing U.S. missiles and increase the Soviet's first-strike capability.

"I think surely that they have gone for the first-strike capability," McFarlane said. "That is different from saying that they intend to execute a first strike for, if you have the capability, you then are able to coerce or blackmail our behavior during a crisis without firing a shot."