American warnings about the possible shipment of advanced aircraft to Nicaragua are part of a broader concern here over the military balance of power in the region.
The mobilization of thousands of Nicaraguan students in recent days underscores not only tensions in the region, but the Sandinista government's general buildup of its armed forces. In the view of officials in the United States, this is considerable, particularly when Soviet and Cuban aid is taken into account.
''The Cuban presence is all pervasive, with upwards of 10,000 personnel spread throughout all levels of both military and governmental organizations,'' Gen. Paul F. Gorman, head of the US Southern Command, based in Panama, told Congress in August. ''Soviet-bloc arms shipments ensure the continued modernization and growth of the Sandinista armed forces, including sufficient T- 55 tanks to outfit an armored brigade, APCs (armored personnel carriers), and larger-caliber artillery and trucks - an unmatched offensive capability in the region.''
Nicaragua has a larger-active duty armed force (61,800) than El Salvador and Honduras combined (58,850). And as a proportion of its population, Nicaragua has an armed force that is more than three times larger than the other two countries. Costa Rica, Nicaragua's neighbor to the south, maintains only a paramilitary force of 2,500 that has no significant weaponry.
US intelligence sources estimate that there is some $300 million in ongoing military construction in Nicaragua, funded by Cuba and the Soviet Union.
According to the International Institute for Strategic Studies (IISS), El Salvador, Guatemala, and Honduras have only a handful of light tanks between them. Nicaragua has at least 60 larger and more-threatening main battle tanks, and is thus presumably better able to mount an attack across borders.
In terms of air power, reports the IISS, Nicaragua is at a decided disadvantage. It has 10 combat aircraft, most of which are converted trainers that have been in service for decades. El Salvador has 59 combat aircraft, including 17 US-built A-37 ''Dragonfly'' strike jets, which have been used against leftist rebels there. With the help of US military aid, El Salvador recently doubled the number of its ''Huey'' assault helicopters, to about 40. Honduras has 30 combat jets, although these are far from top-of-the-line planes.
US officials dispute some of the IISS figures. According to Pentagon estimates, for example, Nicaragua has as many as 150 tanks, along with increasing numbers of anti-aircraft artillery and other weapons.
Nicaraguan officials continue to insist that they have the right to obtain more advanced aircraft for their country's self-defense, and they have made no secret of their desire to obtain them from Warsaw Pact nations, if necessary. Nicaraguan pilots reportedly have received flight training in East European countries.
Nicaragua has been building a runway at a base outside of the capital of Managua that is nearly 13,000 feet long. American military officials say the runway includes fighter revetments.
Thus, the addition of even a small number of advanced aircraft to Nicaragua's arsenal would be of more than symbolic significance.
While the MiG-21 ''Fishbed'' is a relatively venerable plane (it first flew nearly 30 years ago), its capabilities have been refined in combat in Vietnam and the Mideast. It can be armed as an air-to-air fighter with cannons and missiles, or be used to attack ground targets with bombs and rockets. And it has the range to threaten Nicaragua's neighbors.
Nicaragua reportedly has received Soviet-built Mi-24 ''Hind'' assault helicopters of the kind used in Afghanistan. With their rockets and machine guns , these could be more useful than jets against anti-Sandinista rebels. Nicaragua already has the Mi-8 transport version of this helicopter.
US officials do not deny that reconnaissance aircraft are keeping an eye on Nicaraguan port facilities. Sonic booms from such aircraft have added to the tension there.
Officials here continue to vehemently deny that the US is planning to invade Nicaragua. Speculation about such an attack has been fueled by US naval maneuvers involving 25 ships (including the battleship Iowa) now under way in the Caribbean, and by the announcement that airborne infantry units totaling 15, 000 troops will conduct exercises later in Georgia this month.
The war of nerves continued over the weekend, with US Defense Secretary Caspar W. Weinberger's assertion that ''the United States is prepared for a number of contingencies that may have to be taken.'' He did not elaborate, but there have been suggestions that these contingencies might include a naval quarantine of Nicaragua to block further arms shipments.
When asked specifically whether the US might attack any high performance aircraft that might arrive in Nicaragua, the official response is vague, and questioners are referred to President Reagan's press conference last week.
''I'm not going to comment on what might follow, what our procedure would be, '' Mr. Reagan said.