The youth sat in the auditorium of the fortified headquarters of the Salvadoran Army telling reporters that he was a captured Nicaraguan, a Sandinista soldier, ordered here by his government to train Salvadoran guerrillas and fight alongside them.

The presentation last week by Salvadoran officials of the prisoner, Orlando Tardencillac Espinoza Por, 18, reportedly captured Jan. 30, raises the question whether revolutionary Nicaragua has sent men as well as the alleged Soviet Bloc arms into the Salvadoran civil war.

He has been made available to speak to reporters just as the United States, which is supporting the Salvadoran government against leftist opposition forces, is considering sending more military advisers to the Salvadoran government.

One reason being used here and in Washington to justify the increased U.S. military assistance is a fear that "outside personnel" as well as arms will be used to reinforce the faltering insurgents.

Espinoza said he does not know how many others like him are here fighting to overthrow the U.S.-backed government, but he guesses "many."

Espinoza spoke with a pronounced Nicaraguan acent, giving details, including what he said were the addresses of his family in Nicaragua. He said he expected the Nicaraguan government to call him a liar, and that is why he offered every bit of personal information that was requested.

He said all the Salvadoran government had offered him was his life and the "security of my body."

"You can see it's still here," he added.

His statements tended to follow rigorously the Salvadoran government's policy line, but he was more open to questioning than the Sandinistas' imprisoned "counterrevolutionaries" in press conferences staged by the Nicaraguan government in Managua.

When Ezpinoza spoke of the clandestine route he said he took from Managua through the Honduran capital and across the Salvadoran border last spring, he appeared to be speaking from firsthand experience.

He said he had been selected from the ranks of the Sandinista Army and was sent here to train partisans at the Salvadoran national university. Nicaraguan officials have said there might be young Nicaraguans going to El Salvador to fight alongside the guerrillas, but not with government backing.

By Jan 7, Espinoza said, he was commanding a group of about 180 rebels in southern El Salvador who grouped to wait for a planeload of arms that never came. As a result, they saw little action during the failed "final offensive" that began Jan. 10.

On Jan. 30, he said, he was riding a bus when it was stopped by a government patrol. One of his former comrades, he said, turned him in.

Although Espinoza insisted that he came here on the orders of his government, he said he did so "voluntarily and gladly" and that he thought "99 percent" of his comrades in the Sandinista Army would do the same to help the Salvadoran people.