President Reagan has decided to propose to Soviet leader Mikhail Gorbachev in their meetings this week that they agree on a joint statement making a formal commitment to seek reductions in nuclear arsenals, it was learned today.

Reagan intends to press Gorbachev to include reductions to specific levels in the statement, but the president would consider it an important achievement even if the levels are not specified, according to administration sources.

The Soviet reaction is uncertain, especially because Reagan, according to sources, does not intend to offer any limitation on his missile defense program, known as the Strategic Defense Initiative. Such a limitation has been the top priority for Gorbachev.

At the same time, administration spokesmen said that despite intensified Soviet pressure for agreement to continue adhering to the SALT II nuclear arms treaty, Reagan will wait until after the summit to decide what course to take.

National security adviser Robert C. McFarlane said Reagan would take into account several factors, including his meetings Tuesday and Wednesday with Gorbachev, "and reach decisions later in the year."

The question of continuing to honor a previous arms agreement emerged as a central point of U.S.-Soviet debate on the eve of meetings intended to advance the prospects for new accords on arms reductions and other matters.

Suddenly making the issue more visible just before the summit was the unauthorized disclosure of a letter to Reagan from Defense Secretary Caspar W. Weinberger in which he warned against succumbing to "great pressure" in Geneva to make concessions on SALT II. The letter accompanied the first of two Pentagon reports on alleged Soviet treaty violations.

As he walked in the chilly Swiss air on his way to a meeting with senior advisers this afternoon, Reagan was asked by reporters whether he planned to fire Weinberger. "Hell no," he responded, and dismissed a remark by one of his own senior aides the day before that disclosure of the letter was an attempt to sabotage the summit.

Secretary of State George P. Shultz, however, appearing on the ABC-TV program "This Week With David Brinkley," hinted at exasperation. "What does offend me is the lack of discipline within the administration," Shultz said.

Pentagon spokesmen have said that the defense secretary's office had nothing to do with the leak and that it is being investigated.

With the Soviet leader due to arrive here Monday, there were signs of a slightly improved atmosphere in statements from both camps. Soviet spokesman Georgi Arbatov said in an interview that "the atmosphere has slightly improved, maybe because the summit is acquiring its own momentum." Presidential spokesman Larry Speakes said the summit may "serve to provide the highest level of direction and impetus for negotiations in the key areas of our relationship."

White House chief of staff Donald T. Regan, interviewed on the CBS program "Face the Nation" today, said that "an overall agreement in principle on how they want to proceed" on arms control is "a possibility."

The Weinberger letter, which generated a countercurrent to the warming trend, was cited by Arbatov as evidence that Gorbachev was right in asserting that a "military-industrial complex" in the United States largely determines U.S. policy in the Reagan administration. Arbatov said under questioning in a news conference for several hundred international journalists that Weinberger's letter appeared to be "a direct attempt to torpedo the whole arms control process."

McFarlane, while terming disclosure of Weinberger's letter "unfortunate," described the defense secretary's report as a normal part of the governmental decision-making process.

The Soviet Union already has proposed that the superpowers continue for one year their policy of not undercutting the SALT II treaty, which is due to expire Dec. 31. Arbatov, a leading Soviet spokesman, said today that if Reagan and Gorbachev "don't come out with a determination to sustain this regime" of SALT II, the summit will be "a failure."

But senior Reagan administration officials said that the president was not likely to decide at the summit on any policy that goes beyond his decision last June for a conditional and indefinite continuation of the policy of not undercutting the treaty.

McFarlane said that Reagan will consider additional Pentagon findings, the Soviet record on compliance and progress in arms negotiations, as well as his talks with Gorbachev, before making a decision "later in the year."

The Soviet proposal for a one-year extension of the never-ratified SALT II restrictions was made secretly by Foreign Minister Eduard Shevardnadze in late October. Senior U.S. officials said the proposal generated a new round of policy discussions in Washington about how to respond.

Following an internal debate, the decision was made more than a week ago to continue the existing policy of not undercutting the treaty limits for the time being but not to agree to a one-year extension, offcials said.

One factor in the decision, according to officials, is that the United States would be required to dismantle many more nuclear weapons than the Soviets would during the next year to continue adherence to the treaty. But beginning in 1987, according to an administration official, the tables will be turned, and the Soviets would have to dismantle more weapons than the United States would.

During 1986, new U.S. Trident submarines coming into service would force the administration to dismantle at least two older Poseidons with hundreds of nuclear warheads to remain under the SALT II limits. But in later years, the Soviets are planning deployment of new systems that would force them to dismantle a greater number of weapons to keep within the treaty restrictions.

The SALT II issue came up in Moscow during the recent visit by Shultz and is expected to be discussed Tuesday afternoon when Reagan and Gorbachev turn to arms control issues. Shultz said today that Reagan is "not very likely" to reach an agreement with Gorbachev on SALT II, but he would not foreclose that possibility.

Reagan's decision to seek a joint statement of objectives comes after an unsuccessful attempt to win Soviet agreement to such goals when Shultz was in Moscow early this month.

Since then, the administration has agreed on changes in the guidelines that it is seeking to give new impetus to the ongoing nuclear and space weapons negotiations. There is also hope that Reagan, as head of state, may be able to persuade Gorbachev although others have failed.

While diplomatic activity continued on bilateral U.S.-Soviet accords that could be ready for endorsement by Reagan and Gorbachev, McFarlane said there had been no progress during the past few days toward narrowing the wide gaps on nuclear and space arms and areas of geographical conflict.

Gen. Nikolai Chervov, a senior arms control official of the Soviet General Staff, said at a news conference that "a new situation has now come into being" as a result of recent arms control offers by Moscow.

Chervov cited, among other things, a Soviet proposal that the superpowers reduce their land-based intercontinental ballistic missiles by between 200 and 300 in the immediate future as a sign of good faith in the search for deeper cuts.

The Soviet general said that there had been "no reply" from the United States. Shultz and Speakes strongly indicated last week in Washington that the administration has little interest in the plan, which it sees as more advantageous to the Soviet Union than to the United States.

Chervov, in reply to questions, said the Soviet position is "absolutely clear-cut." Even scientific research toward the creation of "space-strike weapons" must be prohibited, he said.

Other Soviet officials, including Gorbachev, have said at times that fundamental research in the space area cannot be stopped.

Asked by reporters about the different versions of Moscow's position on space research, McFarlane said the Soviets "are going to have to duke it out and figure out which one is their position."

McFarlane said there is "substantial agreement" on a joint U.S.-Soviet statement to be made final at the summit setting out "common purposes" of the two nations in the drive against the spread of nuclear weapons to additional nations.

He said that despite lengthy discussions there is yet no agreement on efforts by the two nations to ban chemical weapons globally or to halt the proliferation of chemical weapons.