The first major naval exercise by the North Atlantic Treaty Organization in the Gulf of Mexico and the Straits of Florida, between the U.S. mainland and Cuba, will be held between March 8 and 18, according to Pentagon sources.

Planning for the exercise, which will involve 28 ships and 80 aircraft, mostly operating from airfields on the U.S. mainland, from six NATO nations, is under way at NATO headquarters in Brussels, the sources say.

The exercise, called "Safe Pass," comes at a time when the Reagan administration has begun to call attention publicly to the threat it sees to allied shipping in any wartime emergency from Cuban or Soviet ships, planes and submarines operating in Caribbean or Gulf waters.

Both Defense Secretary Caspar W. Weinberger and the Joint Chiefs of Staff, in their annual reports to Congress this month on the U.S. military posture, have made a direct link between Cuba and the ability of the United States to reinforce its NATO allies in any crisis in Europe.

"In peacetime," Weinberger said in his report to Congress, "44 percent of all foreign trade tonnage and 45 percent of the crude oil imported into the United States pass through the Caribbean. In wartime, half of NATO's supplies would transit by sea from Gulf ports through the Florida straits and onward to Europe."

Because of this, added the Joint Chiefs, "Cuba would pose a significant threat to U.S. crisis-response capabilities."

The Cubans have about 200 Soviet-built Mig fighters that presumably could be used to disrupt allied shipping, plus two Soviet-supplied, diesel-powered attack submarines, a new frigate and some 50 torpedo and missile patrol boats.

The Soviets, the Pentagon also has been pointing out, use Cuba as a major intelligence-gathering center for U.S. military operations on the eastern and Gulf coasts and have greatly expanded their naval presence in the South Atlantic.

The Gulf of Mexico is barely inside NATO's geographical boundaries, which extend southward to the Tropic of Cancer. In the past, the only NATO operations in this region were carried out infrequently by the five or six ships that make up NATO's so-called standing naval force in the Atlantic. This includes single vessels from the United States, Canada, Britain, West Germany and the Netherlands.

These vessels will take part in the "Safe Pass" exercise, along with six more Canadian ships, five more from the Netherlands and nine others from the United States. U.S. military sources say it will mark the first time allied forces have conducted a major exercise in the Gulf.

The exercise will closely follow a 13-ship maneuver involving U.S. and Canadian warships in the same region between Feb. 27 and March 3.