The Reagan administration informed Soviet authorities last week that it is willing to extend its policy of "not undercutting" the 1979 SALT II strategic arms limitation accord, opening the way to a formal agreement along these lines at the summit in Geneva next week.

Meanwhile, President Reagan yesterday modified once again his description of his Strategic Defense Initiative (SDI) in an interview with European broadcasters, telling them the United States should develop a missile defense and provide it to the Soviets "at cost."

Reagan's interview came on a busy day of presummit activity, most conducted by "administration officials" who asked for anonymity. The news that the United States would continue to respect SALT II -- and that Secretary of State George P. Shultz had discussed this with Soviet leaders in Moscow last week -- came at a White House briefing for reporters in the morning.

Another senior official at the briefing said the president intends to practice "quiet diplomacy" when he discusses human rights with Soviet leader Mikhail Gorbachev rather than trying to publicly embarrass the Soviets on the issue.

Reporters were also told that Shultz discussed in Moscow a proposal for a 35-year program of cooperative U.S.-Soviet research on nuclear fusion for civil purposes, another area of possible agreement at the summit. The Soviets are believed to have an advanced program of fusion research, leading the world in some aspects, according to U.S. scientists.

The proposed cooperative program, eventually costing up to $3.5 billion in contributions from both nations, would involve construction of expensive facilities, administration sources said.

More details on yesterday's developments, by subject:

*SALT II. Last June, the Reagan administration decided to dismantle an older Poseidon submarine to stay within the SALT II treaty limits for nuclear weapons when a new Trident submarine comes into service this fall. The decision, initially opposed by the Defense Department, represented a further embrace by the Reagan administration of a treaty that the president called "fatally flawed" during the 1980 presidential campaign.

It was unclear at the time, however, whether the Reagan policy of staying within the limits would continue beyond Dec. 31, when the SALT II treaty would have expired, had it ever been ratified. (Both countries pledged to respect the treaty despite the lack of formal ratification.) Shultz reportedly indicated to Soviet leader Mikhail Gorbachev and Foreign Minister Eduard Shevardnadze that the United States will now entertain extending its willingness to respect the treaty, though other U.S. officials said Reagan has not yet formally approved the idea of further extension.

Reagan in June placed two conditions on continuing not to undercut SALT II: that the Soviet Union "exercise comparable restraint" in its observance of the pact, and that the Soviet Union "actively pursues arms reduction agreements" in the Geneva negotiations. These two conditions were repeated by Shultz last week in presenting the U.S. posture to the Soviets in Moscow, officials said.

In recent months high-ranking U.S. and Soviet officials are reported to have discussed the fate of SALT II on several occasions, including during the New York visit in October of Soviet Foreign Minister Shevardnadze. The Soviets are reported to have proposed a joint statement, in terms different from those used in Washington, calling for continued adherence to the treaty.

Officials said there had been no clear-cut Soviet response to Shultz's statements on SALT II last week. Thus it remains an unresolved issue to be explored by Reagan and Gorbachev next week.

Reagan, in an interview with U.S. News & World Report following Shultz's trip to Moscow last week, said he "probably" would discuss U.S. and Soviet compliance with SALT II with Gorbachev. The president said that continued U.S. offensive-arms restraint under SALT II terms "would be dependent on Soviet restraint also. Obviously we're not going to stand by if we're the only one practicing the restraint."

*The president's interview. In the interview with British, French, Italian, Swiss and West German broadcasters, Reagan reiterated what he called "my dream" of what could happen with a strategic missile defense program.

"We don't start deploying it," he said. "We get everybody together, and we say, 'Here it is' . . . . Now we think that all of us who have nuclear weapons should agree that we're going to eliminate the nuclear weapons. But we will make available to everyone this weapon. I don't mean we'll give it to them. They're going to have to pay for it, but at cost."

The 74-year-old president appeared cheerful throughout the interview and said he looked forward to one-on-one meetings with Gorbachev, 54.

"It'll be the first time we've ever had someone on our side of the table who's older than the fellow on the other side of the table," Reagan said with a smile. "So maybe I can help this young man with some fatherly advice."

The president discounted the likelihood of a joint statement at the end of the summit, saying he is not "a great fan of communiques." But he expressed hope that the two leaders could send "a signal" to arms control negotiators that ultimately would produce a U.S.-Soviet agreement.

In what appeared to be a new statement of policy, Reagan said the United States would engage the Soviets in discussion of a nuclear-free zone in Europe. This proposal has long been advocated by the Soviets and opposed by the United States and its European allies.

Basic military doctrine of the NATO alliance holds that nuclear weapons are needed in Europe to offset the advantage in conventional forces of the Warsaw Pact powers.

White House officials did not respond to questions about whether Reagan had made a new offer or whether he misspoke. Last week, Reagan and his spokesmen issued a series of corrections about the president saying that the SDI would be deployed only after both superpowers had eliminated their nuclear weapons, a declaration that seemed to give the Soviets a veto over the plan.

Asked about his past reference to the Soviet Union as an "evil empire," Reagan replied that the Soviets had referred to his administration as "cannibals."

"So, I think both of us have stopped that language, thinking that we'll get farther at the meetings if we come together to try and eliminate the need for such talk," Reagan said.

*Human rights. In yesterday morning's briefing, a senior official said that the administration had for the time being discarded its strong public criticism on human rights issues in an attempt to use quiet diplomacy to prod the Soviets into a "sincere and earnest" approach. The official also said there would be no attempt made to reach a new agreement on human rights at the summit.

"The Helsinki agreement is enough," he said. "We don't need anything more except compliance by the Soviets with an agreement they have already signed."

The official said that the administration was "hopeful" that the Soviets would agree to allow increases in Jewish emigration and release from prison some dissidents who also want to emigrate.

The official said that the United States has presented a list to the Soviets of separated spouses and families who want to be reunited by leaving the Soviet Union and that Reagan may discuss some specific cases of these families as well as dissidents when he meets Gorbachev.

The issue of Soviet violations of past arms control treaties -- the subject of a special report to be delivered to Reagan today -- was discussed in testimony yesterday to the Senate Foreign Relations Committee by Assistant Secretary of Defense Richard N. Perle.

Perle said the Pentagon had identified an "increasing pattern of violations" of arms pacts by the Soviet Union and said violations have become increasingly serious. Among other things, the Soviets are accused of introducing more "new" missiles than are permitted by the SALT II treaty and encoding missile test data to an excessive extent.

Nonetheless, Perle expressed hope that "we can fix" the violations problem at the Geneva summit and suggested that if that is not completely successful, the United States can formulate "responses proportional to the violations" without abandoning the basic treaties.

Perle warned, however, that "we have to face up to our responsibility" in the face of Soviet violations. If the Soviets fail in the long run to take corrective action, he said, the arms control process "will be destroyed."