At least 1,000 Nicaraguans, disaffected from their nation's Sandinista rulers , are quietly undergoing military training in Costa Rica.
The majority of these Nicaraguans are training in small camps along the sparsely populated Costa Rica-Nicaragua border under former Sandinista revolutionary war hero Eden Pastora, say knowledgeable sources close to these Nicaraguans. Their intention reportedly is to reenter Nicaragua and agitate for change against the Sandinistas.
''Our struggle doesn't have the goal of invading Nicaragua, but of uprising from within,'' said Plutarco Hernandez, a Costa Rican who is a close friend of Mr. Pastora's. The size and nature of the training missions are corroborated by Nicaraguan businessman-politician Alfonso Robelo Callejas, a former Sandinista junta member who now heads an opposition political party, the Nicaraguan Democratic Movement (MDN), from Costa Rica.
Besides the men and women trained by Pastora--who gained fame in the Nicaraguan revolution as ''Commander Zero''--other armed rebel groups, both leftists and rightists, are operating in the border zone.
The Revolutionary Armed Forces of Nicaragua (FARN), led by former Sandinista commander Fernando Chamorro, has between 100 and 200 men, and a rightist group of ''Somocistas,'' many of them former National Guardsmen linked to Nicaragua's overthrown dictator, Anastasio Somoza Debayle, have between 200 and 300 men in Costa Rica, says Mr. Robelo.
Pastora's camps have trained about 1,000 soldiers, Robelo says. He estimates that 2,000 persons are involved in military and political anti-Sandinista activities in Costa Rica.
Until recently, Costa Rica appeared to ignore the anti-Sandinista groups operating within its borders, although Pastora's activities were much discussed in San Jose and were frequent subjects of Costa Rican newspaper reports.
But the country changed course two weeks ago following complaints from the border region that the Nicaraguans were intimidating residents of the area. Other factors in the Costa Rican change of attitude may be Pastora's handling of the defection of 11 Sandinista soldiers, which he turned into a news media event , and reported forays of Nicaraguan soldiers across the border in search of ''counterrevolutionaries.''
After these events, the Costa Rica Security Council announced: ''We will not tolerate, under any circumstances, the alteration of peace by armed groups in our territory.'' Minister of Security Angel Solano ordered 350 civil guards to search for Nicaraguan armed bands and training camps in the border area.
So far the search has resulted in 24 arrests. Costa Rican Civil Guard head Oscar Vidal says 14 of those captured claimed to belong to the Fifteenth of September, a group of former Somoza National Guardsmen. The other 10, he said, are followers of Pastora.
Costa Rican security forces now acknowledge that armed bands circulate on the frontier. They say they have found weapons, communications equipment, military fatigues, and documents, but say they have not as yet found any training camps.
''We've heard rumors that the camps exist, and we're searching for them,'' said Ministry of Security adviser Johnny Campos. ''But we haven't even been in government for a month,'' he says, referring to the new Costa Rican government innaugurated May 8. ''We need some time,'' he said.
Costa Rican President Luis Alberto Monge also had threatened to ask the Organization of American States to send observers to the border to guarantee the nation's neutrality, for Costa Rica has no standing army. But Monge has now declined to call for the observers.
Sources close to the nation's security officials privately say officials began to squirm about two months ago, when Eden Pastora resurfaced in Costa Rica after nine months in exile elsewhere.
Pastora charged the Sandinista government with reneging on promises to support political pluralism, nonalignment, and press freedom, and with following a Cuban-style revolution. He pledged to make the junta mend its ways ''at gunpoint'' if necessary.
Costa Rica had quietly allowed Sandinista revolutionaries to use its frontier as a staging area for strikes into Nicaragua in 1979. But San Jose officials now are unwilling to cooperate with anti-Sandinista forces.
''There are two different governments in 1979 and now,'' explained one security official, referring to the Feb. 7 election that brought in Mr. Monge and ended the government of Rodrigo Carazo. Now, he says, ''our position is totally neutral in relation to the politics of our neighbors. And that's how it will stay.''
Costa Rica has ordered Mr. Pastora to leave the country. Pastora reportedly headed for Honduras, and is said to be organizing resistance among disenchanted Nicaraguans in that country.
For his part, Alfonso Robelo says he has ''had talks with Pastora that should end up in a strong alliance.
''I coincide with his view of the democratic rescue of the Nicaraguan revolution,'' he explained.
The businessman, who moved to Costa Rica with his family in April, did not discard the possibility of establishing an alliance with FARN as well. His MDN is composed largely of middle-class entrepreneurs whose businesses, he says, were strangled by the Sandinistas.
He stresses that he is ''not directly involved in any military activity.'' But he cautions: ''I could support some military action in the future.''
According to several sources, Pastora has several experienced field commanders on his side.
''Pastora's group is not that big yet,'' says Robelo. ''It's just beginning. Pastora's main resources lie within the Sandinista army and popular militia.''
However, Commander Zero is said to hold the respect of a large number of middle-class Nicaraguans as well as most of the 25,000-strong Nicaraguan community in Costa Rica. So far his group has refused to join forces with FARN because of its reported links with the Somocistas.
Hernandez denied that Pastora had accepted money from the United States Central Intelligence Agency, a charge that the Sandinistas have repeatedly made. Yet he admitted that Pastora representatives have met with some members of the US Congress.
According to Hernandez, Pastora's camps usually house about 125 persons. They receive four to 10 weeks of training, depending on their previous military experience. The men and women receive weapons training and political indoctrination, he said.
''Our groups are composed of people of all walks of life, even l4-year-old children,'' he explained. '' Those who have the physical capacity learn to fight. The others help in the camp.''
The soldiers use conventional weapons, such as M-16s, he said. The Pastora aide refused to identify the source of arms or funding, but he said most of the weapons are acquired on the international black market or from within Nicaragua, Hernandez said. Locations of the camps shift regularly to avoid detection by Costa Rican guards.
The Costa Rican government now is trying to polish and reorganize its 7,000 -man security forces. The civil and rural guards, who make up the core of the force, are poorly trained and underequipped. Costa Rica currently is seeking international assistance to change that.