The Senate Armed Services Committee, acting on its own, has authorized adding 100 multiple-warhead Minuteman III missiles to the 550 already deployed, a move that could lead to the collapse of the U.S.-Soviet strategic arms limitation talks.

The prospect that such a move could become what is being called a "SALT breaker" lies in its potential for pushing the United States above 1,200 multiple-warhead-carrying missiles. The 1,200 figure is the ceiling for such weapons in the unratified but not yet abandoned SALT II agreement.

A senior administration official said yesterday, "We are very concerned about it because it will make it much more difficult for both sides to observe SALT."

The committee action was opposed in closed-door hearings by both Defense Secretary Harold Brown and Air Force Chief of Staff Gen. Lew Allen. Administration officials see the move as prmarily an effort by a small number of conservative lawmakers to damage chances for an eventual SALT agreement.

The sponsor of the amendment was Sen. Gordon J. Humphrey (R-N.H.), who takes a hardline view toward Moscow on defense matters.

Humphrey, according to his associates, believes the measure will "send a signal to the Sovies that the United States is reversing its strategic decline." Adding the missiles, he feels, would be a relatively cheap and quick way to try to ensure that more U.S. nuclear warheads would survive a Soviet first strike or would have a better chance of knocking out protected Soviet missiles in a nuclear exchange.

The United States has had 1,000 Minuteman intercontinental ballistic missiles deployed since the early 1970s; 550 of them are the Minuteman III variety. These carry three atomic bombs in their nose and are the most accurate and lethal weapon in the U.S. arsenal. The other 450 Minuteman missiles are of the Minuteman II variety and carry a single warhead.

Humphrey's measure was approved by the committee in a 13-to-4 vote. All seven Republican members backed it, as did six of the 10 Democrats.

The panel's action came in a closed-door session earlier this week as the committee was completing work on the Pentagon's overall $5.19 billion weapons procurement bill for fiscal 1981. But for reasons thus far unexplained, except for one staff member claiming it was an "oversight," this particular action was not included in the detailed public statement of the major committee decisions released Thursday.

Though Minuteman is no longer in production, 128 Minuteman IIIs are in storage and are meant primarily for testing.

Under the committee plan, 100 would be taken out to replace Minuteman II missiles in their silos. Deployment could start by mid-1981. The swap would cost $44 million, the first $10 million of which was authorized by the committee this week.

More multiple-warhead missiles mean that any missiles that survive a Soviet attack would be able to lob more than just one warhead back at the Soviet Union.

Critics argue, however, that the gain in surviving warheads is marginal, that the loss of test or replacement missiles is important and that it doesn't make sense to sink more money and missiles into the same silos that the Pentagon already has testified are becoming increasingly vulnerable to Soviet attack.

As for SALT, if the United States deployed these missiles and continued with the planned deployment of the new Trident missile-firing submarines, by 1983 the United States would either be over the 1,200 ceiling or would have to retire some weapons -- such as additional submarine-launched Poseidon missiles -- that it is not now planning to retire.

Aside from the 550 Minuteman IIIs, the United States has 496 multiple-warhead missiles aboard Poseidon submarines, and the first of 14 already authorized Trident submarines, each of which can carry 24 such missiles, is to begin deployment next year.

For the committee action to take effect, it will have to get approval from both the Senate and the House, which made no similar amendment. It would then have to go through the congressional appropriations process before the money could be spent.

The Minuteman measure was the committee's second controversial move concerning the nation's strategic arsenal.

The panel also approved $1.55 billion to continue development of the mobile MX ICBM, which is meant to be the land-based missile of the future.

The panel endorsed deployment of all 200 planned missiles and construction of an elaborate "shell-game" system of 4,600 concrete shelters among which the missiles would be constantly moved to reduce the chances of their being hit in a surprise attack.

But an amendment allowing only half the silos to be built, at least initially, in Utah and Nevada is troubling the Pentagon. The original plan called for all missiles to be located within a single huge tract of government land in those states.

The amendment, meant to ease public opposition in those states, requires the Air Force to study alternative sites and report back to the committee by Feb. 1.

While the Air Force is looking into possible sites in Texas and New Mexico, both Pentagon and administration officials feel it will not be easy politically to find other places and that, if new sites can be found, it is certain to add at least $3 billion to the $30 billion-to-$40 billion project.

What bothers the Pentagon most is what it sees as ambiguous language, now on the record, that doesn't make it clear what will happen if the Air Force can't find acceptable alternatives.

Some administration officials feel that the amendment, though meant to preserve MX through compromise, may wind up killing it.