The U.S. Navy's biggest ships soon will resume the "show-the-flag" missions off the coast of Nicaragua that were suspended last summer while the Reagan administration tried to defuse Central America as an election issue.

The aircraft carrier USS Nimitz, with its full air wing of 90 warplanes, left Norfolk Tuesday and is steaming toward the Caribbean coast of Central America, Pentagon officials said yesterday. The battleship Iowa and its battle group are to follow in a few weeks.

Neither ship is expected to spend more than a few days in the region, but both visits will be "high-visibility," with officials of allied Central American countries possibly being flown out for shipboard visits. Between the summers of 1983 and 1984, the administration maintained a strong naval presence off both coasts of Central America as a sign of support for Honduras and El Salvador and a warning to the leftist government of Nicaragua, officials said.

The ship visits, like the large land exercises the U.S. Army conducted in Honduras, were suspended as the administration sought to lower its military profile in Central America during the election campaign. Large-scale land exercises also are about to resume, according to Honduran and U.S. officials.

The last big U.S. ships to visit the western Caribbean were the Iowa in August and the carrier USS Kennedy in July, Navy officials said yesterday.

During the intervening months, the Navy kept one ship stationed off the Pacific coast of Central America almost continuously. That ship, a frigate or destroyer reconfigured as an intelligence collector, has been used to eavesdrop on radio conversations in Nicaragua, officials said.

The Nimitz, accompanied by a nuclear-powered cruiser and a frigate, will spend about three days in the western Caribbean and then return to Norfolk, officials said.

A Soviet surface action group has been operating near Cuba for about three weeks. The Soviet fleet is led by a Sovremenny-class destroyer and includes two Krivak-class frigates and a Tango-class diesel submarine.

The Iowa and its battle group, now in port in Norfolk, will follow the Nimitz in early February, officials said.