There is mounting evidence that Soviet MIG fighter jets destined for Nicaragua have been in Cuba for at least a year, and that Fidel Castro is holding up their delivery to Nicaragua.
Western diplomats believe that if such MIG-21s or MIG-23s were ever to be delivered, the United States government would consider the action a serious provocation to which it would have to respond.
Recently the following evidence has come to light:
* A leading Latin American statesman, closely connected to the Socialist International, told the Monitor that high-level Cuban sources informed him last year that some 10 MIGs were in Havana, waiting to be delivered to Managua. But, the Cubans said, Fidel Castro had decided not to send the jets for fear of provoking strong US retaliation against Nicaragua.
This statesman said the Soviets had decided not to go against Castro's decision. He said that the Soviets had left much of the decisionmaking in this instance to Cuba, as was often the case with the East bloc's Central American policy. He said the MIGs had been in Cuba since at least June of last year and to his knowledge they were still there.
* A top Costa Rican official said last week that ''10 months ago'' he was shown pictures of six MIGs by Reagan administration officials. He said the US officials told him these planes were in Cuba and were destined for Nicaragua, but Fidel Castro had decided not to send them on to Managua for the time being.
He also stated that Nicaraguan officials had complained to him last year that Castro was keeping the MIGs in Havana and not sending them on to Nicaragua.
* Within the past six weeks a prominent South American statesman, according to a liberal Latin American politician who had just come out of a meeting with him, was shown new pictures of MIGs by US officials. The statesman was told they showed MIGs in Cuba, destined for Nicaragua. He reportedly did not specify what kind of MIGs they were.
The statesman, according to the Monitor source, was so perturbed by the pictures that he went to see the Soviet ambassador in his country.
Among Western diplomats in Managua there seems to be a consensus that there are MIGs in Havana destined for Nicaragua. Some of these diplomats, however, wonder why the Nicaraguans would want the planes. First, they say, MIG-21s are much too sophisticated for the level of air warfare in the region. Second, as one such diplomat put it, ''To have MIGs in Nicaragua would be to wave a red rag in the face of the US bull.''
Most Western diplomats spoken to here say that if the MIGs actually got to Nicaragua, the US response at the very least would be to try to knock them out with an air strike. This leads some diplomats to wonder whether the Soviets, Cubans, or Nicaraguans ever had any real intention of introducing the MIGs into Nicaragua. Instead, they suspect the intention may be to keep them in Cuba as bargaining chips in any negotiations with the US.
There is also speculation by Western diplomats here and in Costa Rica that this might be a maneuver to emphasize the peacemaking role of Fidel Castro - to make him be seen as the man who stopped the delivery of the MIGs and now wants to negotiate with the US.
But other Western diplomats here speculate that the Nicaraguans seriously think they can get and keep the MIGs. In the last few months Sandinista leaders have repeatedly made public statements about Nicaragua's need for sophisticated airplanes, implying that France or the Soviet Union would be the source of such planes.
France was approached by the Nicaraguan government in April and asked to approve the sale of 18 French Mirage jets, according to a Western diplomat, but decided against approval.
Meanwhile, according to Western diplomats, the Nicaraguans have been building at least two airports that could accommodate MIGs. One is in eastern Nicaragua and was referred to publicly by US President Reagan in 1982. According to US government sources, the construction began toward the end of 1981 but was stopped in 1983 because of financing problems. Western diplomatic sources say construction resumed this spring.
According to non-US Western diplomats from several embassies here, another airport is being constructed on a small triangular peninsula that juts out into Lake Managua directly across from the capital city of Managua. Called Punto Huetes, it is alleged to have a particularly long, 10,000-foot runway and some underground installations. Construction of the airport is supposed to be finished by the end of the year.
If Nicaragua is to fly MIGs, it must have pilots trained to do so. Until now it has not had any. But, according to Western diplomats here, Nicaraguan pilots are being trained in East Europe. A group of more than 20 Nicaraguan pilots, for example, has been training in Bulgaria, these diplomats say. State Department sources say that at least one group has completed its initial training in Bulgaria and the pilots are now in the Soviet Union and Cuba for further training.