For more than a year, the Reagan administration has been contemplating strong U.S. military and political responses to potential escalation of military activity in Central America by Cuba and the Soviet Union.

The proposed reactions under discussion include direct use of U.S. air and naval power, dispatching U.S. Air Force jets to the area to offset Soviet-made jets, large-scale increases in aid to friendly countries and invocation of the 1947 Rio Treaty permitting U.S. participation in collective defense in Latin America.

This is the reality behind the unsolicited warnings last week by Assistant Secretary of State Thomas O. Enders about possible introduction into Nicaragua of "modern fighter aircraft" by Cuba or the Soviet Union "or even Cuban combat troops."

"Clearly a dangerous situation would then develop, unacceptable not only to Central America but to the American nations as a whole. We have communicated to Moscow and Havana how dangerous such a move would be," Enders told the Senate Foreign Relations Committee Tuesday and a House Foreign Affairs subcommittee Thursday.

According to documents made available to The Washington Post and interviews with policy-makers, official attention has been given for many months to the next Central American moves on the superpower chessboard. From the administration's viewpoint, any dramatic change in the Soviet military presence or Cuban military activity close to the United States would be politically unpalatable and detrimental to the overall U.S. strategic position in the world.

Discussion and planning on this topic in the administration has been in the category of preparations for possibilities, known in government argot as "contingency planning." No firm decision about what to do has been made, according to official sources, and under government procedures no decision would be likely until a "contingency" becomes reality.

One of the administration's earliest known actions regarding possible escalation in the area is President Reagan's decision after a National Security Council meeting Nov. 16, 1981, to order development of "contingency plans to deal with unacceptable military actions by Cuba." The same NSC meeting on Central America led to presidential approval of undercover U.S. support for anti-government insurgents in Nicaragua.

The contingency plans, according to an official report of the NSC meeting written at the time, were to address the "possible use of U.S. forces to deter the introduction of Cuban military forces into Central America" and include the possibility of "a petroleum quarantine and/or retaliatory air reaction against Cuban forces and installations."

Presidential approval for such contingency planning was interpreted in policy-making circles as a consolation prize for then-Secretary of State Alexander M. Haig Jr., who reportedly wanted to go much farther and faster toward use of U.S. forces against Cuba and Nicaragua. Reagan, according to an authoritative account, rejected Haig's ideas as too dangerous and lacking in support by the public and U.S. military chiefs.

By April, 1982, five months after the contingency planning order, State Department officials were beginning to believe that "opportunities as well as challenges" could flow from communist escalation in the area.

"Introduction of MiGs into Nicaragua could be exploited to obtain financing for upgrading of Honduran air force and stationing of U.S. squadron in Honduras," said a State Department paper prepared for NSC discussion that month on U.S. policy in Central America and Cuba.

Addressing a second leading possibility, the paper went on to say that "introduction of organic Cuban units into Nicaragua in response to threats to the survival of the Sandinista regime would provide opportunity for invocation of Rio Treaty aimed at producing ultimatum to Cubans to get out . . . or else."

Still another threat, this time without a suggested answer, was that Congress could reject U.S. policy in El Salvador "through cutting off military aid or requiring negotiations as one condition of our semiannual certifications." This was seen as "an enormous, perhaps irretrievable setback" causing loss of all U.S. influence in El Salvador, irresistible pressure in Honduras to reach an accommodation with Nicaragua and an immediate drive in Costa Rica to "find safety through neutralism."

These possibilities are believed to have been addressed in the closely held annex to a document prepared last April for an NSC discussion about trends, strategies and actions in the region for the two years to come. The document was published 10 days ago by The New York Times. Sources said it had been prepared for consideration in the meeting by the president and others rather than reflecting presidential discussion and decisions as described by The Times.

Since 70 Nicaraguan pilots and mechanics were sent for jet training in Bulgaria in early 1980, and especially since intensive lengthening of Nicaraguan airfield runways in 1981, U.S. policy-makers have been scanning the horizon, expecting to see Soviet MiG fighters.

Haig is reported to have taken up the question of MiGs for Nicaragua on several occasions with Soviet officials, including Foreign Minister Andrei A. Gromyko, and an administration official said the same message had been delivered to the Soviets by Haig's successor, George P. Shultz.

In December, 1981, Haig discussed the MiGs with Nicaraguan Foreign Minister Miguel D'Escoto. According to an unofficial but informed account, Haig sternly warned that the United States would take unspecified action if MiGs were brought to Nicaragua, and D'Escoto replied that there were no plans to do so unless his country were attacked.

Despite statements from D'Escoto and other Nicaraguans seeming to forswear the MiGs, U.S. intelligence agencies and their policy chiefs have continued to sound alarms.

"The Soviets may have already decided to raise the ante," said a policy paper last July 12 prepared as the basis of the "Central America overview" to be presented to Reagan at a meeting of the highly secret National Security Planning Group the following day.

The paper, drafted in the State Department and approved by officials at the Defense Department, Central Intelligence Agency and the NSC, reported that a shipment of MiGs was then "en route to Cuba with eventual movement on to Nicaragua a distinct possibility." It went on to say that "the arrival of MiGs in Nicaragua would not only tilt the regional military balance even further, but would be a major political/psychological shock demanding a prompt U.S. countermove."

According to the document, "Actions available to us range from CADC Central American Democratic Community and OAS Organization of American States denunciation, through U.S. air unit interchange with Honduras and Colombia, up to and including action to destroy the planes and/or a blockade quarantine."

The "most promising initial response," the briefing paper said, would include:

"Demonstrations" of U.S. Air Force ability to relocate to Honduras and San Andres, an island belonging to Colombia about 100 miles east of Nicaragua's east coast. It was noted that airport improvements in Honduras, undertaken in a $21 million U.S. military construction program, would be useful in this respect.

Training of Hondurans and Colombians in "maintenance and handling of an appropriate fighter" and funds to provide Honduras "a limited number of the planes."

Political action in the CADC and OAS.

Apart from the issue of communist escalation, the policy paper suggested that U.S. efforts in the region should be strengthened by additional military assistance for Honduras and Costa Rica and by "increased pressure on Nicaragua."

"Growing popular dissatisfaction in Nicaragua and the encouraging, though still modest success of Sandinista hero-turned-dissident Eden Pastora's international activities suggest that this is the time to step up the pressure on Nicaragua," the document said. It made no reference to the administration's undercover support of Nicaraguan insurgents but suggested that increased economic pressure could be applied.

Administration officials said last week that there is no sign of the expected MiGs in Nicaragua, although the Sandinistas reportedly made known last December that 30 of the 70 MiG trainees had returned home from Bulgaria. It is unclear, however, whether they passed their courses. One unofficial report is that only eight of the Nicaraguan pilots could qualify for jets and those only for older and less powerful models.

There is also no indication of Cuban combat units in Nicaragua or of any request for them by Sandinista authorities, according to the administration sources. However, the sources said about 2,000 Cuban military or security personnel in Nicaragua could ease the way for combat forces, if needed.

Dramatic escalation, some officials said, is less likely than a continued gradual tightening of Soviet-Cuban-Nicaraguan links, unless the Sandinista leadership panics under the growing pressures. More likely than MiG fighters, according to some at the Pentagon, is receipt by Nicaragua of jet trainers, something which would make a strong U.S. reaction more difficult.

A prominent academic observer agreed that "there has been a slow but steady incremental deepening" of the Soviet-Nicaraguan relationship in recent months. Robert S. Leiken of the Georgetown Center of Strategic and International Studies, author of a recent monograph on Soviet strategy in Latin America, said the dominant faction in the Sandinista leadership actually appears to be pursuing the relationship with more eagerness and vigor than are the Soviets.

"This is due in part to the conviction in Managua that the United States will not tolerate a revolutionary regime in Central America and that the ultimate natural protection is the Soviet Union," he said.