U.S officials believe that several Soviet T55 heavy tanks recently were shipped secretly to Nicaragua as the first step in a long-rumored plan to equip the Nicaraguan armed forces with Soviet weapons including tanks and Mig jets.

The alleged arrival of the tanks, coupled with what U.S. officials say has been a steadily increasing resumption of arms smuggling through Nicaragua to leftist guerrillas in El Salvador over the last six weeks, has raised the possibility of new tensions between the Reagan administration and the leftist-oriented revolutionary government that has ruled Nicaragua since 1979.

Although, Washington regards Nicaragua's alleged intention to build up its armed forces substantially as a potential source of serious frictions in Central America, U.S. officials said yesterday that the administration does not plan to take action against the Managua government.

Instead, the officials said, U.S. strategy is to seek to persuade the Nicaraguans that their best interests and their hopes for resumed U.S. aid call for avoiding actions regarded as provocative by neighboring countries.

Daniel Ortega, head of Nicaragua's revolutionary junta, told The Washington Post that reports about his country's getting Soviet tanks or jet fighters are "totally unfounded." However, despite his denial, U.S. officials said yesterday that recent intelligence has led them to conclude that some Soviet tanks have arrived in Nicaragua via Cuba.

The officials, who declined to be identified, cited as evidence what they called "reliable inteligence reports" that large pieces of equipment, covered with tarpaulins to prevent identification, have been unloaded in Nicaragua from Cuban transports at night under conditions of heavy secrecy and security.

These officials said they were not able to say with complete certainty that the shipments consisted of tanks. Nor could they specify how many tanks allegedly came into Nicaragua or what has happened to them.

But, the officials added, analysis of the intelligence -- coupled with other information about Nicaraguan defense planning and the knowledge that the Nicaraguan army has built two tank-training courses modeled on the kind used by Soviet and Cuban forces --had led to the tentative conclusion that some Soviet armor is in Nicaragua and that more is likely to follow.

In addition, the officials said, although there is no sign that any Soviet jets have been brought into Nicaragua, there are strong grounds for believing that the Nicaraguans are planning for their arrival. According to the officials, approximately 80 Nicaraguan pilots are being trained in various parts of Eastern Europe to fly the various Mig models, and the Nicaraguans have been broadening and hardening runways at various airfields to handle jets.

Ortega, in a telephone interview with Washington Post special correspondent Alma Guillemoprieto, insisted the landing strips "are being reconditioned to provide better service to an isolated part of the country" and said the aim was to give "better civil, not military, service."

However, U.S. officials countered that the work being done clearly is intended to allow fighter-size jets to use the fields. They added that the fields are in areas where there is no civilian need nor use for jet planes.

Since taking power in a bloody civil war that overthrew longtime dicatator Anastasio Somoza, Nicaragua's revolutionary leaders have stressed repeatedly their intention to build a new "people's army," which already has more tha 20,000 people under arms and is still growing.

The Nicaraguans contend that such a force is required to defend themselves against potential aggression from the rightist, military-dominated governments in neighboring Honduras, El Salvador and Guatemala and to guard against invasion or guerrilla warfare by former Somoza followers and other opponents across the border in Honduras.

In response, U.S. officials say the Nicaraguan forces are larger than any other Central American army. This size, the officials contend, has created a vicious circle that reinforces suspicion of Nicaragua in the other countries and that goes against Nicaragua's own stated aim of protecting itself.

Ortega, in his interview with The Washington Post, said Nicaragua has obtained "defensive" weapons from many countries including the Soviet bloc, but he insisted that none were of a sophisticated, offensive nature such as the tanks, jet fighters and heavy mortars that his goverment reportedly is seeking from the Soviets.

Nonetheless, U.S. officials contend, there have been rumors for months that those elements in the Nicaraguan government who are openly sympathetic to Cuba and the Soviet Union want to build up a formidable, Soviet-equipped force on the Cuban model. Nicaragua's decision last fall to cooperate in what the United States contends was massive Cuban-orchestrated arms aid to the Salvadoran guerrillas was regarded here as a sign that the leftists were winning the struggle for control of the revolutionary government; the last evidence about the Soviet tanks appears, in the view of U.S. analysts, to be a further indication of Nicaraguan movement toward the communist camp.

In reaction against that trend, President Reagan in April cut off further U.S. financial aid to Nicaragua. However, the administration made clear at the time that if it received evidence of Nicaragua halting the arms flow to El Salvador and ceasing other actions aimed against its neighbors, the United States would consider resuming the aid, which is badly needed by the economically hard-pressed Nicaraguan government.

U.S. officials said the administration plans to stick wih that approach despite its concern about an infusion of Soviet weaponry into Nicaragua. This policy of patience, they explained, is due to several factors: Washington's feeling that there is no viable alternative to the revolutionary government at this time, recognition that an internal power struggle is still under way that could tilt the balance back toward more moderate forces within Nicaragua and a companion belief that open U.S. hostility could be used by the radical leftists within the revolutionary government to rally support for their pro-Cuban stance.

"We are offering them a choice," summed up one official. "We're telling them they can have good relations with the United States, or they can follow a path of adventurism toward their neighbors, but they can't have both. If they want our aid and goodwill, they have to choose."