While U.S. observation planes from an American airstrip here keep watch over a guerrilla war in neighboring El Salvador, Honduran officials are nervously keeping watch over another being fought by U.S.-backed rebels along the border with Nicaragua.

The American-led battle to contain leftist insurgencies in Central America has thrust this sparsely populated, impoverished country into an unlikely role as militarized bulwark against the spread of such insurgencies.

It has also produced a topsy-turvy sense here that these bases either do not officially exist, as in the case of the ones concerned with what Nicaragua calls counterrevolutionaries, hence contras, or are given a very low profile belying their true importance, as in the case of the Honduran-owned but U.S.-run facilities.

"Everything is temporary here," said U.S. Army Maj. William Lowe about this mountain-rimmed, newly paved landing field in an isolated valley about 50 miles northwest of Tegucigalpa in central Honduras. He added, "The only permanent structure is the kennel," a cool concrete enclosure for the watchdogs.

The only buildings are scores of wooden Central American Tropical huts, or CATs, that house the 900 U.S. troops based here. The tiny wooden PX does not even carry underwear, so if a soldier here on an exercise forgets his, he's in trouble, Lowe explained.

No one stays for more than 90 or 180 days except the commanding officer, who spends a year in the tropical heat. "So there is no institutional memory," Lowe added, contributing to the idea that this is no permanent operation.

Officially, this base is headquarters for joint U.S.-Honduran exercises and training. But much more goes on here.

Nightly, U.S. Air Force observation planes from the 224th Military Intelligence unit take off in efforts to spot concentrations of left-wing guerrillas in El Salvador, then pass that information to Salvadoran Army commanders. Engineers from the Lear Siegler Co. are here testing a new pilotless drone called Skyeye.

Every few weeks, 150-man companies of U.S. infantry arrive for jungle training, and every year thousands of troops land on the 7,500-foot runway to take part in the Big Pine exercises with the Hondurans.

For the first time, U.S. tanks have arrived. They are to be used in military exercises this week within three miles of the Nicaraguan border. The tanks will simulate a Nicaraguan attack on Honduras.

U.S. civilian officials, more blunt than the military, make clear that the purpose of this base is long-term. It is meant, they say, both to intimidate Nicaragua's rulers and warn them of U.S. abilities to move troops quickly, and as a guarantee to Honduras, which now finds itself indirectly linked to battles in both Nicaragua and El Salvador.

The expanding U.S. military presence has led some critics to label this country as the "USS Honduras," that is, an American aircraft carrier. But the present Honduran government and military thus far appear to welcome that presence because it brings aid and because they are concerned not just about Nicaraguan ambitions but perhaps even more about what they describe as an expansionist, overpopulated and well-armed El Salvador, with which they fought a brief war in 1969.

In any case, the Hondurans are concerned about the U.S.-backed contras. According to Honduran and U.S. estimates, there are 12,000 to 15,000 operating out of base camps along the Honduran-Nicaraguan border, 250 miles east of here.

As the prospect increases that the U.S. Congress will prevent renewal of aid to the contras, alarm is growing almost daily among top Honduran civilian and military officials that the rebels will be forced to fade back into Honduras -- and create a potentially devastating security problem for this country.

U.S. officials say there are now 5,000 to 6,000 contras inside Honduras at any one time, indicating that they have cut back their operations and withdrawn from some forward positions in Nicaragua. Contra leaders interviewed elsewhere say they have now moved into more isolated regions.

"The Hondurans are getting nervous. They are getting exposed. They fear they will bear the onus for some potential conflict with Nicaragua," one experienced U.S. official said. The Hondurans want the suspension of congressional funding lifted mostly so the contras can operate in Nicaragua and not in Honduras, he added.

"It's the constant repairing to the Honduran sanctuary and the potential for cross-border incidents and Nicaraguan retaliation that bothers them," another U.S. official said.

"We clearly would prefer a democratic government in Nicaragua," said a senior Honduran official. "But it is not Honduras' responsibility to pursue democracy in Central America at all costs."

"We ask ourselves," said a Honduran officer, "who will be the ones to deactivate, disarm and control these people" if there is no more U.S. government funding and they retreat entirely into Honduras. "It's like having 12,000 PLO fighters in your country who want a separate state," added a Honduran diplomat.

The Hondurans quietly have supported what initially was secret U.S. policy on the contras for two years now, these officials say. But they see little prospect that the rebels will be successful, see no end in sight, and are critical of what they see as shifting U.S. objectives and explanations.

Furthermore, Hondurans read of opinion polls in the United States and show little confidence that there is public support there for any U.S. military intervention.

Hence, Honduras is seeking to revise a 1954 security pact with Washington to provide a legal guarantee of protection. The chances for U.S. acceptance of such a commitment, however, are said by U.S. officials to be small.

The senior Honduran officer said that his country does not want to jeopardize its U.S. relations and that it does fear Cuban-style subversion. But in the current situation, if the contras are defeated militarily or have their support shut off, he said, the United States has a responsibility to disperse them so Honduras is not locked into permanent problems with Nicaragua.

As the rebel leaders see it, if they get no more U.S. government aid, they can try elsewhere, fight on for 20 years at lower levels or, as one put it, "clear out, move to Australia or Canada, and leave the problem to the other Central American countries and the United States."

Another Honduran official charged that U.S. concern for his country failed to extend beyond the military issues. "The businesses and tourists fear to come because they think we are involved in war. But it is safer here than in New York or Miami," he said.

Some American officials in the region describe the low-budget U.S. bases here and the support for the rebels as the "low-cost option," saying that at relatively little expense or involvement, the United States can intimidate Nicaragua.

Interviews here and in El Salvador yield a view that U.S. military aid and training to the Salvadorans has blunted leftist guerrillas in that country and given the Army the upper hand. Similarly, most officials here say that the contras clearly are a problem for the Sandinistas' rule and that their actions have taken some of the pressure off El Salvador.

But Honduran officials say the contras have also helped the Sandinistas strengthen their grip on the country. U.S. and Latin American specialists say the Soviets and Cubans also have a "low-cost option" in leftist guerrilla movements. They say such groups could trigger a U.S. military intervention that might cause a serious loss of American prestige throughout Latin America and Europe.