The Reagan administration is approaching a crucial decision on whether to take action against Nicaragua to prevent that country from becoming "another Cuba," according to senior officials in several government agencies.

The pressure to act arises from a widely held assessment throughout the top ranks of the administration that Nicaragua is increasingly becoming an armed camp from which leftist revolution can be exported to neighboring El Salvador, Guatemala, Honduras and Costa Rica.

The decision-making process is now coming to a head, one highly placed official said. He forecast that President Reagan, who has been involved in a series of unpublicized discussions with his top national security aides, may address the handful of options again within the next two weeks.

Offering his personal view, the official said, "We are duty bound to do something," without saying what that action would be. Another senior official said: "You will find there will be actions by the United States that will speak for themselves before long. Things have got to be confronted in a variety of ways."

Officials refused to outline specific options under consideration, but it was clear that they view the most urgent problems as military ones: a continuing sizable arms buildup in Nicaragua and the improved fortunes of Cuban-trained guerrilla forces in El Salvador and other nearby states. Like the public remarks of Secretary of State Alexander M. Haig Jr., the private comments of high officials conspicuously left open the possibility that the U.S. response will likewise involve military actions.

Nicaragua has emerged as the nub of a painful set of Central American problems, which have engaged the administration since its earliest weeks. A major new element today is a much greater sense of urgency, a shared belief that the problems are becoming worse and that failure to take action in the near future will only add to the dangers and difficulties to be faced in an inevitable day of reckoning.

The military buildup in Nicaragua, according to administration officials, is aimed at creating a standing army of 45,000 to 50,000 troops, about three times the military force of the overthrown right-wing Nicaraguan leader, Anastasio Somoza. "By Central American standards this is just huge," Undersecretary of State James Buckley said last week.

Nicaraguan pilots are being trained in Bulgaria to fly Soviet-built Mig jets, according to the officials. The pilots are completing their training, the officials said, suggesting that Mig deliveries to Nicaragua may not be far behind. Airfield runways in the country reportedly are being strengthened and extended to handle the new planes. In addition, there is said to be evidence that additional Soviet-built tanks will soon arrive to augment about two dozen reportedly already received.

U.S. officials have been publicly estimating that 1,000 to 1,500 Cuban military advisers, training specialists and security personnel are in Nicaragua. However, sources said there is credible intelligence that the Cuban presence actually may range up to 2,000, although no organized Cuban units are believed inside the country.

What Haig publicly has termed the "totalitarian trend" in Nicaraguan politics is another worrisome element. Among the alarm bells, in the Washington view, are the arrest Oct. 21 of four business leaders who had criticized a "drift toward Marxist-Leninism;" repeated closings of the opposition newspaper, La Prensa, and of radio stations, and the flow of power and authority from moderates to more radical elements in the leadership.

Earlier in the year, Washington explored diplomatic means of reversing the gathering military and political trends in Nicaragua. A mid-August visit to Managua by the State Department's chief of Latin American affairs, Thomas O. Enders, featured blunt talk on both sides, with the Nicaraguan leaders blaming their actions on threats from the United States.

In the Managua meetings, Enders presented what administration sources now describe as a remarkable set of proposals, including something close to a "non-aggression pact" and assurances of control over Nicaraguan exile groups who have been training in U.S. paramilitary camps, in return for a shift in the Nicaraguan government's behavior and orientation.

There was no answer from Managua for nearly a month. Then a series of messages from Nicaragua complaining bitterly about U.S. military exercises in Central America, one of which was used as a justification for national mobilization, was taken in Washington as a negative response. Although Managua has continued to signal that it wishes to continue the political dialogue, the military buildup and internal crackdown there appear to have convinced the Reagan administration that the negotiating track has failed.

Haig testified last March that "the seizure of Nicaragua" had been accomplished as the first result of a communist "hit list" in Central America. Later he backed away from this assertion. The present assessment, according to an official source, is: "We have not concluded that Nicaragua is totally gone. But they are very close. The hour is very late."

The cumulative effect of the military and political developments is growing concern in Washington about the possibility of "a general conflagration" in Central America. "Nicaragua's neighbors are getting scared," said an administration source. He suggested that private opinion about Nicaragua is beginning to change among Latin leaders, including those in such countries as Mexico and Venezuela who were among the early supporters of the Sandinista revolution.

There is striking unanimity of view within the White House, State and Defense departments and Central Intelligence Agency about the situation in Nicaragua and the associated problems elsewhere in Central America. But this unanimity does not extend to the thorny question of what to do about it.

Haig, before congressional committees, has steadfastly refused to rule out attempts to overthrow the leftist regime in Nicaragua. Haig's tough talk has been interpreted in part as an effort to intimidate the Nicaraguans into restraining their activities without the necessity for U.S. military action.

Beyond this, Haig is known to have asked the Pentagon to study military options. The Pentagon, however, is less than enthusiastic about the effectiveness of possible military actions in the region.

A naval blockade to interdict arms flowing to Central America, which is considered to be among the most practical of the military options, was given short shrift in public as late as last Wednesday by Secretary of Defense Caspar W. Weinberger. He said a blockade did not appear to be necessary at this time but did not rule it out for the future.

"What you are really seeing" in the differing reactions, said one top official, "is the range of views within the government and the institutional concerns." Because there are no really good military options, he said, "the military is very nervous about the potentially disastrous impact" of some possible decisions.

What worries the military leadership, he explained, is that controversial and possibly ineffective action in Central America might destroy the public consensus for rebuilding military strength, and also draw forces and resources away from areas that the military considers more essential. "So it is also a priorities problem," he said. "If there is any lesson of Vietnam it is to make sure that the public understands with clarity what the problems are."

The involvement of forces from other Latin countries is among the options being studied, but officials said that for any joint military efforts to be effective, the United States must be involved.

If the United States moves to interdict the flow of arms or sources of training for Nicaragua, then a larger question is how the Cubans--who have very sizable military forces--and the Russians--who have a huge investment in Fidel Castro--will react.

In a strategic sense, the threatened breakup of the U.S. sphere of influence on its periphery in Central America is parallel in many respects to the Soviet Union's troubles on its periphery in Poland and Eastern Europe. And the two situations are linked in official calculations.

Washington has been concerned that a military move in Central America would provide the Russians with the pretext for military action in Poland. Conversely, a high point of concern last July that the Soviets might intervene in Poland generated intensified discussion here of a U.S. military move in Central America, on the theory that the Russians would be too heavily involved in their own problems to respond.

In addition to its political and military importance, Nicaragua is important in a symbolic sense, some officials here believe. Fidel Castro's efforts to export his Cuban revolution were unsuccessful during the 1960s and most of the 1970s. The coming to power and now the leftward swing of Nicaragua's Sandinistas are seen here to reflect the first clear victory for Castroism in two decades.

This is all the more worrisome to Washington officials for two reasons:

First, Cuba and the Soviet Union are reported to have unified their previously discordant ideological and operational policies in the hemisphere in the past several years. A resounding success in Nicaragua would seem to be validation of these unified policies.

Second, Nicaragua's geographical position gives it more leverage for easy extension than that of Cuba. Nicaragua is not an island, but a large country in the center of the Central American land mass. Should it some day establish training camps for guerrillas such as those which have long existed in western Cuba, Nicaragua could be a much more effective base for the spread of revolution.