Aeroflot's flights between the Soviet Union and the United States may be limited to two a week next year unless U.S. airlines are given a chance to carry passengers to the Soveit Union for the 1980 Olympics, the Civil Aeronautics Board indicated yesterday.

The CAB's caveat came in an order tha gives Aeroflot, the Soviet Union's state-owned airline the authority to operate three flights a week from Moscow to the United States through the winter. But the board specifically set an end to that special permission for March 31, 1980. That is the date Aeroflot wants to begin operating again the four flights a week it usually runs in the peak travel period.

Implicit in the board's order is the expectation tha Aeroflot will be allowed to boost its number of flights next spring if the Soviet Union assures the United States that U.S. airlines will be given equal opportunities to carry passengers to the Soviet Union, especially during the Olympics.

"We continue to be concerned that U.S. carriers wishing to provide service between the U.S. and the U.S.S.R. be assured fair and equal opportunities," the CAB said in an order signed by Sanford Rederer, director of its bureau of international aviation. Noting that "realistic market access" includes that various ancillary services necessary to conduct business, such as hotel facilities for passengers and crew, resonable office space, employe housing, and necessary ground transportation, the CAB said that "present conditions and operating limitations unduly constrict operations by U.S. carriers."

A government source said yesterday the board order should not be construed as a warning to the Soviet Union but is a way of making sure that U.S. airlines are treated fairly. With the pull-out of Pan American World Airways from Moscow, the U.S. has no regularly scheduled flag carrier service but does have some charter services operating, some of which have encountered administrative problems in the Soviet Union.

Aeroflot currently has authority to operate two weekly roundtrip flights to the United States under a 1974 permit issued by the CAB and approved by the President. But in recent years, extra flights had been authorized by exchange of diplomatic notes between the two governments. This allowed Aeroflot to operate three flights weekly in the winter season (November through March) and four flights weekly the rest of year.

The notes expired on March 31, 1979 because, according to the CAB, the "level of comity and reciprocity on which they were based had not been maintained." In order to avoid disruption of air service between the two nations, however, the CAB granted Aeroflot a seven-month exemption to continue to operate the increased schedule.

At the time, the Soviet Union agreed that simplified charter application precedures and guarantees of approval would be available to all U.S. airlines wishing to operate charters between the two countries. The Soviet Union also gave assurances that satisfactory arrangements would be made for U.S. airlines to particpate in both scheduled and charter programs arranged by U.S. tour operators for the 1980 Olympics.