The Reagan administration is considering reopening portions of the Naval Air Station at Key West for use as an intelligence-gathering center to keep track of Cuban and Soviet military activity in the Caribbean, according to Pentagon sources.

The potential action, which is said to have considerable support within the Pentagon and the administration, is part of an expanding effort to beef up U.S. intelligence capabilities in the Caribbean and Central America and to improve the position of U.S. forces should a showdown develop in the region with Cuba or the Havana-backed government in Nicaragua.

Key West, 90 miles from Cuba and once a bustling Navy air, sea and submarine base, has declined sharply in use over the past several years.

But now, officials say, the first of a new class of missile-carrying hydrofoil patrol boats is based there, and five more will be home-ported in Key West by this fall. These small, fast vessels, each of which carries eight Harpoon antiship missiles, are the only ships currently planned for Key West.

However, sources say the Navy is considering reopening an area big enough to hold six destroyers, and also is studying the possibility of eventually basing a destroyer group at Key West to beef up U.S. antisubmarine warfare capabilities in the waters around Cuba. The Cubans have two diesel-powered Soviet-built "Foxtrot" class submarines, a new Koni class Soviet frigate and about 50 missile or torpedo equipped patrol boats.

While such a threat seems small in comparison with the U.S. Atlantic Fleet, based farther to the north, Defense Secretary Caspar W. Weinberger and other officials have repeatedly called attention in recent public statements to the threat in a wartime emergency the Cuban navy could present to the roughly 45 percent of U.S. shipping and oil-delivery vessels that move through the Caribbean or out of Gulf ports.

Last November, as tensions increased with Cuba, the administration announced it was forming a new Caribbean Command and basing the headquarters at Key West. There are no such troops, however, based there.

The area being considered for reopening is the Truman annex, a 76-acre portion of the air station that has been closed and listed as excess property for several years.

Asked about the site yesterday, a Navy spokesman issued a statement that said "the secretary of the Navy has determined that strategic and training requirements may necessitate retention of excess Truman annex property."

The Navy, the statement said, has requested that the General Services Administration "place a 180-day hold on disposal" of the property.

Key West once again has become strategically important because of the "political turmoil" in the region and the "increasing interference of Cuba and the Soviet Union in regional affairs and heightened U.S. concern," the Navy said.

Meanwhile, another contingent of about 450 soldiers of the El Salvador army arrived yesterday for training by Green Berets at Fort Bragg, N.C.

The soldiers are all recruits and part of a 1,000-man battalion being trained there. In addition, about 400 officer-cadets of the Salvadoran army are being trained at Fort Benning, Ga.

The American training will try to introduce some discipline and skills into the Salvadoran army, which has been accused of a tendency to kill indiscriminately.

American military advisers have already trained one 1,000-man battalion in El Salvandor, but it was felt that bringing the units to Fort Bragg would ensure that their training is complete. In El Salvador, troops and officers were frequently called away from training if they were needed for a skirmish.

Officials say the El Salvador government plans on adding another four or five battalions to its 16,000-man regular army, and the tentative plan is to have each new batch of recruits trained by Americans from the start either here or in El Salvador.

The government troops face a guerilla force estimated to include 5,000-6,000 battle-tested cadre, with perhaps twice that number in various supporting roles.