MIAMI, JAN. 2 -- Late last year Nicaraguan military officers began writing friendly letters to rebel field commanders fighting inside the country, chatting with them by radio and offering them property and other enticements to lay down their arms and accept a Central American peace accord.

The Sandinista officers took their appeals straight to the rebels, known as contras, who were waging the war in Nicaragua, bypassing their top military leaders based in Honduras.

But in a show of corps discipline that surprised even the contras' highest political leaders, no important contra field commander broke ranks to take the Sandinistas up on their offers, according to contra and Sandinista leaders.

The unity the midlevel military leaders showed in responding to the government cease-fire approaches was one sign of a new-found military prowess the six-year-old, 9,000-fighter contra army has displayed in the last three months, at precisely the time when its U.S. aid is most threatened by widespread political support for the regional peace plan.

The contras showed they are capable of rewarding the assistance from Washington, a total of $111 million since mid-1986, by improving their performance in key areas such as field morale and operational and political control.

But their flat rejection of the Sandinista overtures also suggests that the chances for the cease-fire mandated by the peace agreement, signed Aug. 7 in Guatemala by the five Central American presidents, are slim. Less than two weeks remain until the presidents are to meet again Jan. 15 in Costa Rica to evaluate the plan's results.

In interviews in Miami and Santo Domingo, the Dominican Republic's capital and the site of two rounds of unproductive cease-fire talks between the contras and the Sandinistas late last year, contra fighters and political leaders detailed the Sandinistas' efforts to win them over.

The cease-fire campaign "was the most dramatic and difficult test of discipline we have faced," said contra political leader Adolfo Calero. "When it started I thought there might be more breaches."

Sandinista leaders have admitted they were disappointed with the results of their strategy. In a speech Nov. 5, the first deadline for compliance with the peace pact, President Daniel Ortega said he was closing down three small cease-fire zones created a month earlier because "only about 600" contras had used them to accept an amnesty and return to civilian life.

Calero said the total of contra fighters who laid down their rifles since Aug. 7 is "closer to 60." He said only one field commander, in charge of about 60 fighters, accepted the amnesty, but his guerrilla troops did not follow his lead.

The peace plan, among other points, calls for a cease-fire and an amnesty allowing rebel combatants to renounce armed warfare and return to their homes. Sandinista leaders first created the cease-fire zones, which covered 560 square miles of backwoods. A month later they canceled them, saying they would confront the contras with "clubs and lead" on the battlefield while undertaking cease-fire negotiations through a mediator, Cardinal Miguel Obando y Bravo.

Contra leaders asserted that Washington's aid, and the continuous air resupply flights deep inside Nicaragua that it financed, were a central factor allowing them to hold their ranks during the Sandinista campaign. The contras on the ground deferred to their top leaders to make policy decisions about the cease-fire.

The leaders happily admitted that they used the Sandinista cease-fire zones for a series of aerial supply drops.

The smoothly flowing logistical pipeline kept the contras well fed and armed, reportedly encouraging new volunteers to join after a recruitment lull nearly two years long. A veteran contra comandante who goes by the name "Jhonson" said his unit lost no fighters to the amnesty and had a net gain of about 200 since August.

Jhonson said his first message from the Sandinistas was a crumpled, cryptic, hand-scrawled note dated Sept. 28 and signed by Enrique Lopez, a second lieutenant in an outpost called Kuskawas in central Matagalpa province. A peasant courier for the contras slipped into Jhonson's mountainside campsite and handed it to him.

"Your envoys have arranged a cease-fire with me and I agreed to send them medicines and a doctor," the note reads, in a photocopy made available to reporters. Up to then there had been no envoys and no arrangement, Jhonson recalled. Jhonson, whose real name is Luis Fley Gonzalez, heads the contras' 15th of September Regional Command, with more than 1,000 fighters. The unusual spelling of his nom de guerre is just "a whim," he said.

On Oct. 3 came another feeler from Lopez.

"The cease-fire agreement in this area is approved at the highest Sandinista levels . . . . It's not a game," Lopez scribbled. He ended on this tantalizing phrase: "I reiterate my identification with your forces. I think we can do something here." Jhonson also learned then that his younger brother, Lt. Enrique Fley, would be a member of the Sandinista negotiating party.

"It was a stunt to try to split us from our superior commanders. So I set a trap," Jhonson said.

He invited the Sandinistas to a meeting Oct. 10 in a Matagalpa forest hamlet called Bilampi. His brother Enrique, who resembles him down to a curly mustache and goatee, emerged from the bush to say he had come on a "family mission," said Jhonson, who produced a photograph of the two brothers at the camp.

"I told him there were better ways to conduct family business," Jhonson said. He said he took his brother and Lopez prisoner and ordered his intelligence officer to interrogate them. Six days later he released them, apparently unharmed.

In another episode, Sandinista intelligence officer Capt. Bony Rivas sent a two-page typewritten letter to contra commander Raymundo Chavarria, "Cornelio," who was operating near the Matagalpa town of La Dalia. Rivas sent his "offer you can't refuse" in the hands of Cornelio's aging peasant mother. In bold print it says: "IF YOU WANT LAND TO FARM, THE {Sandinista} REVOLUTION WILL GUARANTEE YOU IT."

It made "zero impact," said another contra comandante called "Tono," Walter Calderon. Until recently Tono was the chief of a special sabotage unit of 1,200 fighters based in the La Dalia region.

Laughing uproariously as he recounted the tale during an interview in Santo Domingo, Tono said all Rivas got for his pains was a day-long battle with Cornelio's guerrillas.

Tono himself received queries from Sandinista Army Capt. Miguel Castro, he said, in a zone called Bocaycito in northern Jinotega province. Castro also opened by hinting he might cross over to the contras. But the probe flopped.

"He infiltrated my side with spies and I infiltrated his side. There was never any trust," Tono said.

According to Calero, a comandante named "Quiche," head of the Jorge Salazar Regional Command in southern Nicaragua, reported from the field that he had two direct conversations by military radio in October and November with Sandinista Defense Minister Humberto Ortega.

Calero said Ortega offered Quiche an officer's post in the Sandinista Army if he would accept the amnesty and defect. He refused.

Jhonson and Tono, both members of the contras' cease-fire negotiating team, said they doubted any cease-fire can result from the talks through Obando.

"There's no way a cease-fire will solve the problem of Sandinista Nicaragua," Jhonson said.