President Reagan said in an interview with Soviet journalists published yesterday that the deployment of his proposed space-based missile defense system would come only after an agreement to eliminate offensive nuclear missiles.

The president appeared to be adding a major new element to the U.S. plan for his Strategic Defense Initiative, which is now in the research phase. Previously, Reagan has said he hoped to make nuclear weapons obsolete with the missile defense. In his new remarks, Reagan suggested that he is making elimination of the weapons a condition for deployment of defensive weapons.

Moreover, Reagan expanded on his earlier pledge to share the technology with the Soviets by saying "the terms for getting it" would be "the elimination of the offensive weapons."

White House spokesman Larry Speakes said yesterday Reagan was describing publicly an idea he had mentioned privately to aides in recent weeks. In the past, attempting to describe his vision of strategic defense, Reagan has often extended his rhetoric beyond the policies of his administration and the reality of its research program. One official said Reagan's latest comments were simply "the most pristine" version of his hope for a world without nuclear weapons.

Reagan's remarks came in a 42-minute session with four Soviet journalists in the Oval Office last Thursday. Parts of the interview were published yesterday by the Soviet government newspaper Izvestia, the first Soviet interview of an American president in 24 years.

The interview was published as Secretary of State George P. Shultz arrived in Moscow to begin working out final arrangements for Reagan's Nov. 19-20 meeting in Geneva with Soviet leader Mikhail Gorbachev.

The White House had sought the interview to put Reagan's views before the Soviet people before the summit. Requests for a presummit appearance by Reagan on Soviet television were ignored, U.S. officials said.

Although actual deployment of the missile defense system is many years in the future, the U.S. plans for it have been a point of intense controversy and figure prominently in superpower arms negotiations. The Soviets have attempted to block the program from going beyond research, and Reagan defended it vigorously in the interview.

Reagan has previously said he would talk to the Soviets and the allies before deployment of missile defenses, but never before had he suggested that offensive nuclear missiles would have to be eliminated prior to deployment.

Questioned about this yesterday, Speakes said the president was referring to a "transition period" for negotiations before deployment of the missile defense system, negotiations U.S. officials had previously promised. Speakes said Reagan was discussing a goal "to do away with all offensive weapons and end up with everybody sharing a star shield or nuclear shield."

In the interview, Reagan said, "We would not deploy . . . until we sit down with the other nations of the world, and those that have nuclear arsenals, and see if we cannot come to an agreement on which there will be deployment only if there is elimination of the nuclear weapons."

Reagan said he would make this point to Gorbachev.

"We won't put this weapon, or this system in place, this defensive system, until we do away with our nuclear missiles, our offensive missiles," he said. "But we will make it available to other countries, including the Soviet Union, to do the same thing."

"And if the Soviet Union and the United States both say we will eliminate our offensive weapons, we will put in this defensive thing in case some place in the world a madman some day tries to create these weapons again," he said.

Reagan compared strategic defense to the decision to outlaw poison gas after World War I, after which "we all kept our gas masks. Well, this weapon, if such can be developed, would be today's gas mask," he said.

In response to a question, Reagan acknowledged criticism that a strategic defense system, coupled with offensive missiles, could put one nation "in a position where they might be more likely to dare a first strike."

But Reagan said this would not be the case if the technology is shared, and "I can assure you we are not going to try and monopolize this . . . for a first-strike capability."

Reagan broached the idea of sharing "Star Wars" technology with Moscow last year in a debate with Walter F. Mondale, and has often repeated his hope of giving the results of the $26-billion U.S. research effort to the Soviets at some point.

But there has been criticism of this idea. For example, the congressional Office of Technology Assessment noted in a recent report that if the plans or actual hardware is shared, "potential adversaries" could figure out how to circumvent it. And if the technology is shared, the OTA said, "the American advantage which had enabled us to develop that technology first would necessarily be compromised."

On other topics, Reagan criticized Soviet behavior in Afghanistan, describing the use of bombs shaped like toys that drop from Soviet planes and then blow up when children pick them up. At the end of the interview, one of the Soviet journalists attempted to ask Reagan about this, but was cut off by Speakes.

Asked to reconcile his criticism of Soviet use of force with U.S. involvement in Vietnam, Reagan defended American actions in the war and said, "At no time did we try for victory. Maybe that's what was wrong. We simply tried to maintain a demilitarized zone between North and South Vietnam. And we know the result that has occurred now." That result was, "We left South Vietnam, and North Vietnam swept down, conquered the country . . . in violation of a treaty," he said.

Defending the U.S. invasion of Grenada, Reagan said American forces responded to a call for help and then went home. "And there is the contrast: the Soviet troops have been in Afghanistan for six years now, fighting all that time.