Secretary of Defense Harold Brown said Yesterday that American positions in the strategic arms limitation talks (SALT) are not "cast in concrete," and he gave some details of the latest Soviet proposal that the United States turned down last week.

Unlike President Carter's national security affairs adviser, Zbigniew Brzezinski, who last Sunday declared in a television interview that the Soviet Union would have to accept all outstanding American proposals, Brown said yesterday "we have recast" U.S. offers "a number of times and I would not bar recasting them again."

An aide to Brown said later his position was really close Brzezinski's because Brown saw very little practical room to change U.S. positions, but his public formulation was much more forthcoming than Brzezinski's last week.

Brown indicated - and other officials confirmed - that the latest Soviet offer proposed banning all testing or deployment of completely new types of intercontinental land-based missiles until 1985 by both countries.

The new Soviet offer - presented by Foreign Minister Andrei A. Gromyko to Carter a week ago Saturday - did not cover the difficult question of how much each country would be allowed to modernize existing missile systems during the life of a new SALT agreement.

Thus far the two countries' positions are far apart on this question. The United States feels that the latest Soviet definition of modernization would allow Moscow to put into use several essentially new rocket systems based on existing models that it had recently been testing.

Gromyko's offer prompted widely varying reactions when it became known inside the American national security bureaucracy, authoritative sources said yesterday.

Officials who defended the decision to turn down Gromyko's offer said yesterday it was actually a little worse than previous Soviet positions.

According to this reasoning the Soviets were simply trying to block the United States from testing or deploying a new land-based missile system like the multibillion-dollar MX, now envisioned as an underground mobile missile, while permitting the Soviets to make substantial improvements in their missile force under the guise of "modernization."

Some senior U.S. officials were offended by a remark reportedly made by Gromyko when he was told last Wednesday that the United States regarded his proposal as inadequate. At the same time, the American side declined to make a counterproposal.At that, Gromyko said the U.S. posture would look unreasonable when it became publicly known, several sources said.

This convinced some American officials that the Soviet offer was made for public relations purposes.

At th opposite extreme, some U.S. officials thought the Gromyko proposal offered a significant new opportunity for a breakthrough in the talks, sources said.

According to this view, the idea of a ban on new rockets until 1985 could become the basis for an excellent agreement, provided it were accompanied by the right kind of agreed definitions of "modernization." Some officials apparently thought the United States could have made progress by accepting the Gromyko proposal conditional on a successful outcome of the negotiating on those definitions.

This view was rejected by the administration.

The turndown of Gromyko's proposal was cited by some officials last week was cited by some officials last week as an indication that the Carter administration had, in effect, frozen the SALT negotiations. Other officials said the administration's calculation that this summer would be a politically inopportune time to sign a SALT agreement effectively precluded early progress toward an overall SALT pact.

Carter personally denounced an article in Friday's Washington Post that reported that the administration had effectively frozen the SALT negotiations.

The U.S. chose an unforthcoming way to turn down the Gromyko idea, simply telling him it wasn't good enough, without offering new ideas from the American side. Some senior officials see this as a sign that the administration's posture on SALT now is to stall the talks and preclude any chance of a new agreement this summer.

Other officials said it was just "good negotiating tactics" to turn Gromyko down cold, assuming that the Soviets already have a more forthcoming position on this issue that the turndown should elicit in the weeks or months ahead.

Asked why there had been no U.S. counteroffer yesterday, one senior administration official said the United States can return to these issues in the expert-level SALT negotiations in Geneva.

Several weeks ago, administration sources spoke of the Gromkyo visit here as an opportunity to resolve the major issues on SALT and pave the way for a summer summit between Carter and his Soviet counterpart, Leonid L. Brezhnev. But the administration has since decided not to press for a summer summit, authoritative sources have said.

One problem with the Gromyko offer in the view of some U.S. officials is that it would prohibit the United States from building or deploying the MX system. Other officials say this isn't so serious, since MX is an impractical idea that would cost billions of dollars and probably will never be authorized anyhow.

But MX serves a political purpose in the SALT situation as well. The potential to build MX has already been used by the administration as an argument in defense of a SALT agreement now - an argument that may appeal to a number of the senators whose support would be crucial to win ratification of SALT. An agreement that precludes MX, then, might be harder to sell in the Senate.

Gromyko's proposal is extremely close to a U.S. suggestion made to the Soviets in March last year as part of the administration's new, "comprehension proposal" for SALT. The Soviets turned that approach down cold, forcing the negotiations back onto the path followed in the Ford administration.

A senior official noted this irony yesterday, but said times had changed in the 15 intervening months. The Soviets have completed testing of a lot of new missiles, the official said, creating a need for the United States to match its improved force.

The last U.S. proposal on new types of missiles was to ban them for the life of a protocol intended to accompany a new SALT treaty that would probably last for three years. This would not block the MX, since that weapon now exists only on paper and could not be prepared for testing within three years.

The Soviets' previous position was to accept that U.S. idea but to allow themselves one exception - to test and deploy a new single-warhead rocket system.

Brown said yesterday on "Face the Nation" (CBS, WTOP) that any SALT proposal that prevented the United States from modernizing its land-based intercontinental ballistic missile force with a new missie system of some kind "would have to carry with it on the other side a great many restrictions in order to constitute a fair trade. Something which simply keeps the Soviets from building one missile while allowing them to change and modernize a good many others is not a fair trade for our only planned new land-based missile."