The Pentagon is unveiling elements of a strategy for a would-be Soviet confrontation in the Persian Gulf that includes rushing troops to the region as a "tripwire" force and the possible use of tactical nuclear weapons as a last resort.

Signals are also being sent that if the Soviets confront the United States directly in an area such as the Persian Gulf -- where the United States clearly is militarily vulnerable -- then Moscow could face U.S. actions in other regions where the Soviets might have trouble responding.

The Defense Department's willingness to make known additional details about military options, apparently with White House approval, reflects the political and military dilemma that the Carter administration has found itself in since the Soviet invasion of Afghanistan in late December.

The administration, trying to look tough in an area of Soviet military preponderance, has clearly been stung by doubts voiced openly in Congress about whether the United States has the muscle to back up the president's pledge to defend the Persian Gulf against any Soviet move to control the region.

The administration is now trying to make it known that the United States does have some military actions it could take quickly, while at the same time not hiding the fact that U.S. defenses need beefing up.

At a highly unusual and detailed briefing for reporters at the Pentagon late Thursday, a senior defense official, who cannot be identified under the rules of the briefing, said the United States could put 24,000 men into the region in about two weeks.

The first element, a 1,000-man Army battalion based in Italy, could be in the gulf region in 24 hours armed with antitank weapons and mortars, he said, with the first battalion from the 82nd Airborne Division arriving from the United States a day later.

A Marine airborne contingent of 1,300 men could get there in 48 hours, with 2,200 other Marines coming by sea from the Mediterranean within nine days.

A full Marine brigade of 12,000 men and their equipment could arrive, he said, within 16 days, about the same time it would take to airlift an 8,000-man heavily mechanized Army brigade from the United States with tanks and armored personnel carriers.

Though these troops would be no match over the long run for about 80,000 to 90,000 Soviet troops in nine division that are near the Iranian border in the trans-caucasus region, U.S. officials say the difficulties the Russians would encounter in moving armored forces across 1,000 miles of rugged terrain into the gulf region, or the time it would take, should not be underestimated.

Time is a key factor. Other top defense officials said yesterday that if the United States could get even relatively small ground forces into position before the Soviets got into the area, then it would be the Russians who would face the decision of whether to have a head-on confrontation with U.S. troops.

This is the tripwire strategy. Defense Secretary Harold Brown has warned several times in recent days during Capitol Hill testimony that neither side, in such a confrontation, could be sure of the outcome, and that it would be unwise for anyone to think that a war between the United States and the Soviet Union could be won by either side.

The Soviets also have seven airborne divisions, but officials say that long distances would have to be covered and that the Soviets probably couldn't move more than one division very fast by air.

Brown's warnings before Congress are also said to reflect, in a more veiled way than was spelled out to reporters, the prospect of U.S. counterattacks in other regions or even nuclear warfare.

Asked at the briefing on the Persian Gulf crisis if the United States had thought about eventual escalation to tactical nuclear warfare, the defense official said, "Yes, we are thinking about theater nuclear options in other areas than NATO," meaning outside Western Europe.

Later, other top officials cautioned against making too much out of those remarks, saying the use of these weapons is always implicit in any actual battle with the Russians.

Aside from two aircraft carrier task forces now in the Indian Ocean, the Pentagon says, it could move one or two more squadrons of jet fighters into the region within 36 hours. This could help thwart a Soviet airborne operation.

"So we can do something," said the top defense official, who is well known for his enthusiasm. "We can do, even today, one helluva lot more than some people seem to think. We can't do everything, but we'll be able to do more in a year and a lot more in two or three years. You've got to keep in mind that the Russians can't do everything, any more than we can."

He said that the United States is negotiating to buy or lease 11 commeercial ships, giving it the capability to preposition equipment for an entire armored brigade in the Indian Ocean later this year.

Privately, a number of senior State Department and Defense Department officials said that, despite the seriousness of the situation, they did not expect the Soviets to expand their invasion into the gulf region.

Though officials did not say what bases the U.S. forces would operate from, they indicated that if a country were threatened it would presumably allow U.S. forces in to defend it. Some of the nations in question have considerable American supplied stockpiles that might be used.

On Capitol Hill yesterday, Brown warned that denigration of U.S. military capabilities such as has been heard there recently could cause the Soviets "to underestimate our will and intentions."

Sen. John Tower (R.-Tex.) countered that Soviet intelligence is good enough to know U.S. strengths and "it is a greater hazard to gloss over our deficiences."

In a related development, the Senate met in closed session yesterday at the request of Sen. John Warner (R-Va.) to discuss what he called "totally unacceptable" levels of retaining career military personnel and how to improve them.

The session was closed because some of the information was classified.

Warner predicted that the Senate will vote Monday to approve one of two proposals to reduce loss from the military -- especially of skilled and experienced personnel -- by increasing compensation.