The following dispatch was written by the Monitor's Latin America correspondent James Nelson Goodsell and United Nations correspondent Louis Wiznitzer
Cuba has a bad case of the jitters about the United States. Rhetoric aside, it is not at all clear what action the US is contemplating to have triggered such anxiety in the Caribbean island.
But there are several signs of Cuban agitation over a perceived US threat:
* President Fidel Castro has declared a state of emergency throughout Cuba and called up reserves.
* Cuban Air Force training missions have been stepped up and the number of Air Force spotter planes in the skies doubled.
* Antiaircraft guns are being mounted atop buildings in downtown Havana.
* Coastal defense units have been beefed up and civilians close by mobilized into spotter teams watching the coastlines.
* Loudspeakers in major cities blare martial music in an appeal for support of the mobilization efforts.
These measures are part of the Cubans' hasty response to growing concern that the US is about to take some kind of military action against them. President Castro says he is convinced such action is likely.
At the UN, Cuba has protested the ''impending threat'' against its sovereignty in a letter to Secretary-General Kurt Waldheim and has alerted several key delegations in the Security Council.
UN sources say Secretary of State Alexander M. Haig Jr. recently discussed the ''Cuban involvement in Salvador'' with several allied foreign ministers. In particular he is said to have exploded against reports of 500 Cuban soldiers in El Salvador.
In an interview with the Monitor, Cuba's ambassador to the UN, Raul Roa-Kouri , categorically stated: ''There is not one single Cuban soldier in Salvador. We challenge the American government to prove its assertion or to admit that it was a fabrication.''
Another Cuban diplomat at the UN says: ''Cuba has not been involved in any military aid to the opposition forces in Salvador since the beginning of this year.''
Another diplomatic source, unfriendly neither to Cuba nor to the US says: ''There seems to be evidence of American military preparations for some kind of action in the region.''
Washington appears to be doing little to deny the possibility that it might be contemplating military action.
In fact, Reagan administration officials, starting with Mr. Haig, said last week they have drafted ''extensive studies'' aimed at thwarting Cuban influence in the Western Hemisphere.
These developments, taken with Secretary Haig's refusal last week to rule out possible US action to overthrow or destabilize the Sandinista government in Nicaragua, have produced more speculation about US action against Cuba than at any time since the days of the Cuban missile crisis.
Cubanologists speculate that underlying the concern in Havana over possible US moves is fear that in the event of a US action, Cuba's Soviet allies would not be too quick to respond. The Soviet Union's preoccupation with Eastern Europe, and particularly Poland, the continuing struggle in Afghanistan, and the deep distrust of China are thought in Havana possibly to preclude effective Soviet aid to Cuba. The recent Soviet submarine miscue with Sweden has reportedly been watched with dismay in Havana.
''The Russians will be hard put to stretch themselves across the Atlantic in some emergency,'' comments a highly placed Cuban.
At the same time, the presence of a brigade of Soviet troops on the island is noted by Cuba specialists who wonder whether Washington is taking their existence into consideration if a military strike of any importance is being planned against the island.
Cubanologists claim that, among its plans, the US has a tight sea and air blockade of the island and bombing raids against key military and economic installations on the island on the drawing boards.
But the studies to which Mr. Haig alludes are thought to be less advanced than they would seem at first glance. The rhetoric coming from the administration appears to be ahead of the planning.
No matter how far the planning has gone, however, Cuba and the US appear to be on a new collision course. Washington observers fear the result could be a clash of more than just words.
Moreover, even if the planning for possible action against Cuba is not too advanced, the US has taken a number of steps in recent weeks which suggest to Cuba specialists that there is something in the wind. US naval units in the Caribbean have been augmented, the naval base at Guantanamo on the southeast coast of Cuba strengthened, and spy flights over Cuba increased. Joint US and Latin American naval maneuvers in the Caribbean took place last month. And US military officials met recently with their Latin American counterparts in a session that suggested the possibility the US might be seeking broaded consensus for some action against Cuba.
The Cubans are taking US statements and hints of retaliatory actions such as the Haig comments on Nicaragua seriously.
Cuba's military preparedness, meanwhile, is causing strains on the Cuban public as the cost of these measures translates into new scarcities and privations for an already hard-pressed home front. Dr. Castro recently called for ''greater sacrifices'' in the face of ''the Yankee menace.''
Some administration spokesmen appear to gloat over these developments. A top US official last week said, ''We've got the Cubans nervous and that is a good place for them to be.'' Ever since Roanld Reagan took office last January, Mr. Haig and others have talked somewhat loosely of going to ''the source'' of revolutionary unrest in Central America - the source being understood as Cuba.
Now this rhetoric has become less guarded. Top policy planners talk freely of taking measures aimed at countering alleged Cuban military assistance to both the Sandinista government in Nicaragua and leftist guerrillas seeking to topple the government in El Salvador. Cuba flatly denies US assertions that Cuban troops are present in both countries.