MANAGUA, NICARAGUA, NOV. 3 -- Defense Minister Gen. Humberto Ortega, saying he felt "burned," today described the Oct. 25 defection to the United States of Maj. Roger Miranda Bengoechea as the most important act of treason against the Sandinista Army since its creation in 1979.

Ortega's blunt and sometimes emotion-filled comments in a hastily summoned press conference confirmed that Miranda was one of the defense minister's closest aides and could provide a wealth of intelligence about Sandinista military policies to U.S. officials.

"This was an action caused by deep resentments and personal ambitions which we failed to detect, by a man who turned himself directly over to our enemies at a delicate time, knowing he would be manipulated by them. It is the most important betrayal our Army has had -- the only one of its kind," Ortega said.

Sandinista officers believe Miranda cooperated with the CIA for a short time before defecting.

A U.S. Embassy spokesman in Managua had no comment today on Miranda's desertion. He reportedly has turned himself over to U.S. custody.

Top Sandinista military officials said they called the press conference to preempt what they guess will be the propaganda uses U.S. officials will make of Miranda's information.

Miranda, a personal secretary to the minister, left Nicaragua Oct. 25 for Mexico City, where he visited the U.S. Embassy twice, Ortega said.

Miranda flew from Mexico to the United States, where, on Oct. 28 he placed a phone call from an unidentified city to another Sandinista Army officer in Managua inviting him to defect as well.

Miranda promised the second officer the CIA would be at his Managua home in 15 minutes to start arrangements to evacuate the officer and his family immediately, Ortega said. The officer refused.

Miranda "had access to important military information and documents, because he moved around," Ortega said.

He carried photocopies of some of them to the United States, including plans of the air force, the artillery brigade and other Managua installations, Ortega said.

The defector, a Sandinista militant since 1978, had served as the liaison between Ortega and his general staff as well as with other government agencies, admitted Ortega, Nicaragua's highest military commander. He is the brother of Nicaraguan President Daniel Ortega.

However, Miranda had no access to counterintelligence codes and procedures, which might have revealed how the Sandinista military spies on U.S. activities in the region, Ortega said.

Sandinista officers asserted that Miranda's first contacts with CIA agents came during a week-long trip last September to the United States, when he accompanied Nicaraguan First Lady Rosario Murillo on a hospital visit to Brian Wilson, an antiwar activist injured by a U.S. military train during a protest. Miranda also went to see a 15-year-old daughter who lives in the United States.

After that, Sandinista officials now believe, Miranda began to accumulate documents and data.

Ortega alleged that the "traitor Miranda" became disaffected when he was not promoted to lieutenant colonel in recent Army-wide promotions, and when Ortega demanded more and better quality work from the major.

But a Sandinista officer who knew Miranda well said his colleagues were totally taken aback by his departure. "His work morale never changed in the slightest to the end," the officer said.

Ortega asserted that much of the information Miranda holds is already familiar to the CIA through satellite and spy plane photography.

He added: "Haven't we already said countless times that we defend our right to arm ourselves to the teeth to defend against a U.S. aggression? Haven't we said we want to arm hundreds of thousands ... up to 600,000 Nicaraguans if we can?"

Ortega warned that the Reagan administration will attempt to "manipulate" Miranda's information to undermine a regional peace process which will pass its first major deadline Nov. 5.