San Jose, Costa Rica — Costa Rica's good but ambivalent relations with the United States have come under strain. The difficulty centers around the US role in the guerrilla war against the ruling Sandinistas in neighboring Nicaragua. In particular, the Costa Ricans are upset about the use of DC-3 planes, allegedly by the Central Intelligence Agency , to supply anti-Sandinista guerrillas on Costa Rican territory.
Last week, a top-level Costa Rican government official told the Monitor that ''within the last week or so'' the Costa Rican government had caught such a DC-3 plane supplying Nicaraguan guerrillas in Costa Rica with light ''piranha'' river boats. He said the Costa Rican authorities believed that the plane was being flown by the CIA and that they had subsequently protested to the US government.
The Costa Rican government, the official said, has told the United States that Costa Rica reserved the right to shoot down any aircraft violating its air space - ''albeit with bows and arrows.''
Costa Rican press reports in the last few days say the government has ordered its security forces to shoot at any ''suspicious'' looking airplanes flying over the northern part of the country, which borders Nicaragua.
President Monge is currently visiting Western Europe. According to top Costa Rican officials here, one of the main purposes of his trip has been to talk to leaders of fellow social democratic countries, such as Spain, France, Italy, and Sweden. His aim, they say, is to persuade them to give his country greater financial and political backing, enabling him to distance himself from US policy.
Perhaps in line with this attempt to put Reagan administration policies a little more at arm's length, President Monge met secretly at the French Embassy in London last week with one of the nine members of Nicaragua's ruling Sandinista directorate, Bayardo Arce Castano, according to Nicaraguan Sandinista sources.
However, the difficulties inherent in distancing himself from Washington were illustrated when President Monge reportedly had to put through a call to Ronald Reagan from Rome, urgently asking the US President to unfreeze some $70 million in aid which was being held up.
''Why,'' asks Luis Burstin, a leading Costa Rican commentator and more conservative member of Monge's own party, ''could Monge not get the money from his fellow Socialist International leaders like (French President Francois) Mitterrand or (West German Social Democratic Party chairman) Willy Brandt. Why did he have to go running to Reagan?''
One of the Monge administration's most pressing problems is how to deal with the anti-Sandinista Nicaraguan guerrillas who cross back and forth between northern Costa Rica and southern Nicaragua. Almost daily President Monge must consider how far Costa Rica should and can go in cracking down on the guerrillas.
Since its 1948 revolution, Costa Rica neither has had nor wished to have an Army. Yet, without militarizing, it is almost impossible for Costa Rica to control the Nicaraguan guerrillas.
Costa Rica has only 1,000 men stationed along its 200-mile border with Nicaragua. Top Costa Rican officials admit the guerrillas cross the border with relative ease and sometimes train and get their supplies in Costa Rican territory. But they stress the guerrillas' main base of supplies remains in southern Nicaragua.
The United States role in the Nicaraguan insurgency further complicates the issue for Costa Rica. A top-ranking Costa Rican official says there is some pressure from the US which tends to discourage Costa Rican efforts to curb Nicaraguan guerrilla activities. The official describes the pressure as ''unofficial rather than official, and coming from some parts of the Reagan administration more than others.''
The official says the CIA has consistently ignored Costa Rican neutrality when supplying the Nicaraguan guerrillas, and he cites the DC-3 incident as an example of this.
He and several other high-ranking Costa Rican officials also characterized US Ambassador Curtin Winsor Jr. as tactless in both his private and public statements - although he added that Winsor had been more reserved of late.
He also said that President Reagan himself was said by President Monge to have shown ''more understanding for Costa Rica's policy of neutrality in recent months.''
However, the consensus among many foreign diplomats and Costa Rican officials was that, although the Costa Rican government could not control Nicaraguan guerrilla activities even if it tried its best to do so, it was not cracking down on them as hard as it could.