A senior Reagan administration official yesterday made public for the first time details of the Soviet proposal at the secret U.S.-Soviet talks in Geneva on limiting intermediate-range missiles in Europe.

In a speech in Los Angeles, Eugene V. Rostow, director of the U.S. Arms Control and Disarmament Agency, disclosed that a Soviet draft proposal would limit the number of missile launchers on both sides to 300, with up to 900 warheads, meaning each missile could carry three nuclear bombs.

But the big catch is that Moscow is insisting that British and French missiles -- of which there are 162 based on submarines and on land -- be counted in the U.S. total. This essentially would rule out the planned deployment of 572 new American-built Pershing II and cruise missiles in Europe, while allowing the Soviets to retain virtually all of their new, triple-warhead SS20 missiles, 315 of which are deployed.

On Aug. 1 The Washington Post reported on the Soviet proposal, but Rostow's speech marked the first official public disclosure on details of the confidential talks.

Rostow acknowledged this pledge of confidentiality in his speech, and said he had prepared his talk "with full respect for the rules." But he said the administration also had made clear that the U.S. government has "the duty to keep our people fully informed about the broad principles which govern our approach to the negotiations, and the way they are developing." Rostow described the Soviet proposal as a "wholly one-sided" approach that amounts to "a Soviet demand for hegemony" that "will never be acceptable to us."

Nevertheless, he claimed that "much progress" has been made since the talks began in November, 1981, in terms of "sorting out what is important to each side and illuminating the way to possible solutions." He said a "serious atmosphere" has developed and that "it is clear that a potentiality exists for accommodating the analytic concepts used by both sides." The negotiations are now in recess, and are to resume next month.

Intermediate-range missiles and aircraft, also covered in the talks, are generally those with ranges between 1,000 and 3,000 miles.

Under the Soviet proposal, Moscow would be limited to 300 missiles only in the European portion of the Soviet Union, with an unlimited number in the far eastern part of the Soviet landmass. Because the SS20 is a mobile missile, the United States has called for dismantling all of them, no matter where they are based, in return for a pledge not to deploy the new cruise and Pershing missiles.

Rostow said the Soviet treaty also would have the effect "of forcing the almost total withdrawal from Europe" of so-called U.S. dual-capable aircraft, meaning those able to carry conventional or atomic weapons, "while not affecting most Soviet dual-capable aircraft."

Again, he said, this was partly due to a Soviet demand that French and British forces be counted, ruled out by the United States and allies.