The United States, in response to a 3-month-old-proposal by Soviet President Yuri V. Andropov, has told Moscow that it is prepared to hold bilateral talks on anti-missile defense systems in space, but only within the context of two arms control forums in Geneva, according to government sources.

Shortly after President Reagan's "Star Wars" speech last March, Andropov called for a special meeting of Soviet and American scientists to "discuss the possible consequences of the development of a large-scale, anti-missile defense system."

In a message delivered to the Soviets within the past two weeks, officials said the United States was willing to discuss space defense weapons at the strategic arms reduction talks (START) or at the standing consultative committee set up to police the 1972 anti-ballistic missile (ABM) treaty. Both forums are in Geneva.

In either case, sources said, the American delegations "would be augmented by experts including scientists."

In a related area, it was learned that the Reagan administration has dropped earlier objections and shortly will announce its willingness to participate in a working group in the U.N.'s Geneva-based Committee on Disarmament to study the legal background for a proposed Soviet treaty to ban all weapons in outer space.

Although these two moves indicate a new desire by the administration to discuss weapons control in outer space with the Soviets, sources close to the decisions said yesterday that they were made more to meet pressure from Congress than in expectation of concrete results.

At the Pentagon, however, at least one top Air Force official believes the administration has set in motion events "that could lead to the end of the Reagan 'Star Wars' program."

Sources said that at first U.S. officials ignored the Andropov suggestion for a meeting of scientists, considering it a propaganda ploy. The suggestion originally had been made in a statement that attacked the Reagan space plan as "adventurism" and "speaking of defense while in reality they are planting a mine under the entire process of limiting strategic arms."

Because the Soviet leader has repeated the proposal, as has Soviet Foreign Minister Andrei A. Gromyko, U.S. officials decided a response had to be made.

During interagency discussions over the past month, some Reagan administration officials wanted to "stonewall" the Andropov suggestion, a participant in the discussions said last week. Others, however, wanted to take it up.

A sticking point among U.S. officials was over the question of which scientists, from government or private industry, could be used at an arms talks forum. Several of the leading American experts in space weapons, such as Richard Garwin of the IBM laboratories, have been outspoken critics of the Reagan program.

There appears to be an informal agreement that only government scientists would participate, if the Soviets reply favorably to the U.S. suggestion, sources said.

As of yesterday, a State Department official said, there has been no reply from the Soviets.