Soviet Defense Minister Sergei Sokolov said today that if the United States goes ahead with its space-based defense system, the Soviet Union has "no choice but to take reply measures."

Sokolov, in a lengthy interview with the Soviet news agency Tass published today, said those measures "could be . . . both in the sphere of defensive and offensive armaments," and will be those deemed in the Soviet Union's interests, "not those that Washington leaders would like to incline us to."

In the Tass interview, his first major policy comments since taking over as defense minister in December, Sokolov pointed out that the Soviet Union already has a base of outer space research work but that so far it was not weapons research.

Instead, he said, Soviet research to date is "linked with perfection of space early warning reconnaissance, communication, navigation systems. We are not creating strike space weapons and antimissile defense," he said.

Implicit in his comments was a suggestion that Moscow could move quickly to the weapons phase.

Other Soviet military and civilian figures have said the Soviet Union would keep pace with the American Space Defense Initiative (SDI), with emphasis on possible offensive reply measures.

For the past year, the SDI program, popularly known as "Star Wars," has been Moscow's prime target in both its propaganda and at Geneva during the first round of U.S.-Soviet arms control negotiations this spring.

Soviet officials argue that the program would destabilize the existing nuclear balance, and some also suspect that in pushing for the expensive, highly sophisticated SDI project, Washington hopes to force Moscow into a race that will tax severely its resources and technological capability.

Sokolov repeated the Soviet position that the American emphasis on SDI's defensive capability is a "cover-up" for the deployment of a "new class of weapons -- strike space weapons."

"From the military viewpoint, the American 'Star Wars' plan is an inseparable component part of the U.S. nuclear strategy, the first-strike strategy," he said. "The real meaning of the plan is to get a possibility of a nuclear attack with impunity, to ensure conditions for constant nuclear blackmail of the Soviet Union and other countries."

Arguments that SDI would free the world of nuclear weapons were "outright demagoguery," Sokolov said, and "Washington's excuses" that research for the program is allowed under the 1972 Antiballistic Missile (ABM) treaty are "unconvincing."

He disputed U.S. arguments that the military use of space already has begun. "Communication, navigation, missile attack warning and other satellites . . . are not strike space weapons," he said.

U.S. officials have said that the Soviets for years have had a system to destroy satellites in space, and have charged that new Soviet radars and laser research suggested Moscow was working on an ABM system. In a speech last month, Defense Secretary Caspar W. Weinberger spoke of "the ominous possibility of a deliberate, rapid, unilateral Soviet deployment of strategic defenses."

Sokolov gave a detailed summary of other Soviet positions, including numbers disputing the American claim that the freeze on medium-range nuclear missiles announced by Soviet leader Mikhail Gorbachev on April 7 left the Soviets with nuclear superiority in Europe.

He also rejected allegations that the moratorium is not being observed. He said the Soviet Union has not added "a single missile or a single plane to its intermediate-range forces in the European part of the country" since the moratorium began.