The steamy, forested jungles of Panama hide their presence - but last week more than 6,000 United States soldiers and a similar number of Panamanian National Guardsmen were involved in exercises to test their individual mettle and their ability to work together.
The stated purpose of the joint maneuvers is the defense of the Panama Canal, the 51-mile waterway that bisects the country and provides the vital link between the Atlantic and Pacific oceans. However, US military commanders involved emphasize they ''cannot ignore the security threats of Central America.''
''Indeed,'' says one US colonel involved in the maneuvers, ''those threats make these maneuvers so vital.''
The more modest joint maneuvers in early February between Honduran forces and some 1,600 US troops on Honduras soil, with additional US support personnel in Panama, were part and parcel of the same preoccupation with security. Together, they highlight the continuing and, in some cases, growing US military role throughout Central America.
For the Reagan administration, Central America has clearly become a vortex of US military strategy and thinking. It started with former Secretary of State Alexander Haig Jr.'s attempt to ''draw the line'' in El Salvador. Alarmed at the Nicaraguan -Sandinis- tas' sharp shift leftward, their military buildup, and the allegedly Cuban-orchestrated supply of weapons to revolutionary groups elsewhere in the region, Haig pushed for a military as well as diplomatic-political response.
In five years, US military aid to Central America went from $23 million to about $220 million yearly. Military advisers were flown into El Salvador and Honduras. New funds were provided to train some local forces in the US, in the US ''School of the Americas'' in Panama, or in their home countries. US warships moved near the Central American coast.
Honduras, meanwhile, is the focus of an active effort to unsettle the Sandinista regime in Nicaragua. Reports circulate of CIA help in coordinating hit-and-run, cross-border raids from Honduras into Nicaragua mainly by former National Guardsmen of the ousted, and later murdered, Nicaraguan leader Anastasio Somoza Debayle.
It all added up to a cautious escalation of the US military role in the region, with US advisers and other military personnel very much in evidence in both Honduras and civil-war wracked El Salvador.
''They are the point men for a US military presence based in Panama,'' comments a US diplomat in Panama.
The Reagan administration, however, faces mounting criticism of this buildup. In December Congress responded to reports of CIA involvement in raids against Nicaragua by renewing a ban on support for military attempts to overthrow the Nicaraguan government. Some congressmen see the military aid escalation in general as a prelude to a Vietnam-style US involvement in Central America. Others argue that, in the case of El Salvador, US aid props up a government with an abysmal human-rights record.
US involvement in Central America is not new. US Marines occupied Nicaragua on two occasions in this century. US forces have been in Panama for nearly a century, based largely in the former Panama Canal Zone, which surrounded the Panama Canal until 1980. New canal treaties, which provide for the gradual diminution of the US role in the canal and gradual ''Panamization,'' provide for a long continuation of the US military presence. This week's maneuvers, like those of past years, are part of that continuing presence.
At the same time, however, the Reagan administration believes the US must be ''intimately involved,'' as a State Department source puts it, in Central America as a whole. The region lies too close to the US, he says, to be ignored. And while the rhetoric of US involvement in the region has cooled markedly under Secretary of State George P. Shultz, there has been no visible reduction in the actual US military role.
Mr. Shultz, however, said in strong remarks last week before the House Foreign Affairs Committee that Salvadorean insurgents were ''creating hell'' with Soviet supplied weaponry. He said ''no dice!'' to letting them ''shoot their way into (the Salvadorean) government.''
The current US military involvement includes:
* Some 12,000 US soldiers, almost all of whom are based in Panama. Half of the Panama-based troops have been actively engaged in last week's joint maneuvers there; some of them were engaged in the earlier, similar joint exercises in Honduras. There also are beefed-up US military advisory teams in El Salvador (55 advisers), Honduras (25 trainers), and Costa Rica (10-20 trainers).
* Increased US training of Central American military forces in Panama, in the US, and in their home countries. While the number being trained in Panama at the School of the Americas is down, some 6,000 Salvadorean soldiers have been trained in their home country, and 4,000 Hondurans have been trained in their country. Also, some 2,000 Salvadorean soldiers have received training in the US in the past two years at places like Fort Bragg in North Carolina.
* Stepped up US military assistance, totaling about $400 million in the 1982- 84 period - especially to El Salvador and Honduras. Ten years ago, military aid to these two countries totaled $15 million. Current aid ranges from $81 million in military assistance to El Salvador this year to $2 million in military gear for Costa Rica's low-key Civil Guard.
A country-by-country rundown of US aid shows:
El Salvador: $81 million in 1982, $166 million in 1983, and a requested $180 million in 1984 for military equipment ranging from helicopters to field radios, from nighttime-use rifle scopes to sonar devices that detect boats bringing in equipment by sea for guerrillas. Field training in El Salvador and the US for at least 6,000 members of the country's 18,000-member Army. At the moment, there are 55 US officers and soldiers serving in various advisory roles in El Salvador. Training in the US so far has cost $15.5 million.
Honduras: $40 million in the 1982-83 period (an estimated $25 million in '83 alone) with $40 million requested for 1984, mostly for small arms, helicopters, and other conventional warfare material. In addition, there are approximately 25 US officers and soldiers attached to various Honduran Army units as trainers. Also, upwards of 150 CIA operatives are thought to be in Honduras. US funds reportedly are involved in constructing a major air base near the Nicaraguan border.
Guatemala: $6.36 million for spare parts, radios, and other equipment for the Guatemalan Air Force, mostly to revitalize US-made helicopters. This aid, announced in early January, ends a five-year ban on aid to Guatemala because of human-rights violations.
Costa Rica: $2 million for ''nonlethal'' infantry gear - uniforms, radios, basic field equipment - and field training in Costa Rica for 400 members of Costa Rica's Civil Guard. Some 10 to 20 US officers and soldiers are conducting this training, most of them coming from US bases in Panama.
Panama: No military assistance is currently given the Panamanian National Guard, but the 12,000 US soldiers normally in Panama at various bases offer training and other assistance to the guard.
Nicaragua: No military aid or training.
Belize: No military aid or training is provided, but the small British force in the former colony has US equipment. The US may eventually help Belize to build a civil guard. US MILITARY ROLE IN CENTRAL AMERICA IN 1983 BELIZE * No aid GUATEMALA * 5-year ban on aid ended Jan. 7 * $6.3 million in spare parts for helicopters, other equipment HONDURAS * Joint US-Honduran maneuvers Feb. 1-6 -- 1,600 US troops involved * 25 US advisers * $25 million (est.) in small arms, helicopters, other war material * Helping to build air base near Nicaragua border * Unknown number of CIA operatives (perhaps 150) * Training for troops in Honduras EL SALVADOR * 55 US advisers * $166 million in equipment ranging from rifles to helicopters * Training for troops in El Salvador and US NICARAGUA * No aid COSTA RICA * $2 million in 'nonlethal' equipment * Training for Civil Guard in Costa Rica PANAMA * Joint US-Panama maneuvers Feb. 14-19 -- 6,000 US troops involved * 12,000 US soldiers at US bases * Training for National Guard * School of the Americas (military)