Cuban troops said to be in Salvador; Washington response is low key, but specter of more US involvement looms

The specter of a United States military presence in El Salvador looms amid unconfirmed reports that more than 100 Cuban soldiers have joined leftist guerrillas there.

Some of these reports put the number of Cubans at between 500 and 600, said to be members of Cuba's elite ''quick strike'' force under the nominal control of Interior Minister Ramiro Valdes Menendez.

If the reports are true, they present the US with a major new dilemma in the struggle for control of war-wracked El Salvador.

There is no doubt that a Cuban presence would escalate the Salvadoran struggle both militarily and politically. It certainly would increase tensions between Washington and Havana and could lead to the dispatch of US troops to El Salvador.

US officials are playing down the reports, which have been circulating throughout Central America since the weekend, but also studying them carefully.

Officials of the joint military-civilian junta in El Salvador say, ''There is good evidence that the guerrillas are now benefiting from on-the-scene Cuban advice as well as their earlier training in Cuba.''

Salvadoran government spokesmen suggest the Cubans may have been involved in blowing up the Puente del Oro, a major bridge over the Lempa River, which bisects El Salvador. The destruction of the bridge Oct. 15 required an expertise with explosives not previously exhibited by the guerrillas. With the bridge gone , travel from San Salvador to the eastern third of the country is difficult.

Salvadoran officials are aware that this eastern third of their nation borders on Nicaragua, whose strongly pro-Cuban government served as the avenue for some of the Cuban and Soviet arms that reached the guerrillas late last year.

The reports of Cuban troops in El Salvador indicate they traveled there via Nicaragua.

The reports disagree as to when the troops are supposed to have arrived; some reports say mid-September, others, early October. But all these reports come to the same conclusion: If Cubans are present with the guerrillas, the long Salvadoran civil war has a new dimension.

That war has gone on now for more than three years - and the fortunes of both sides have ebbed and flowed.

For the past year, however, the struggle between the US-backed junta and the Cuban-backed guerrillas has been stalemated. The guerrillas' so-called ''final offensive'' last January fizzled, and it looked for a time as if the government might be getting the upper hand. But events since have suggested otherwise.

Although the guerrillas have been unable to defeat the Salvadoran military on the battlefield, they have managed to keep the soldiers at bay and create mayhem throughout the countryside.

And meanwhile, at least 25,000 Salvadorans have lost their lives in the past two years alone. Many of these deaths did not take place in combat, however, and human-rights groups the world over lay much of the blame on the military and rightist paramilitary groups that are accused of indiscriminate killings.

But there are reports the guerrillas have also have stepped up indiscriminate killings in recent months.