Despite its concern that Nicaragua could become a second Cuba, the Carter administration has decided on a conciliatory approach aimed at influencing the new Sandinista-dominated government toward moderation and friendly relations with the United States.

As described by reliable sources, this strategy involves waiting for the new regime to demonstrate how it intends to govern Nicaragua rather than prejuding it on the basis of the avowedly Marxist and pro-Cuban attitudes of many Sandinista leaders.

"The aim," as summarized by one source, "is to be patient and open-minded and to judge them not by what they've said in the past but by what they do in the future."

To demonstrate its good will, the sources said, the United States is prepared to increase its humanitarian aid to victims of the Nicaragua civil war and, if asked by the governing junta there, to contribute sizable amounts of financial and technical assistance toward reconstructing the strife-torn country.

That offer will be conveyed to the junta by the U.S. ambassador-designate, Lawrence Pezzullo, when he returns to Managua Thursday or Friday, the sources said. The State Department, responding to the junta's formal request for a sign of U.S. recognition, announced yesterday tht Pezzullo is going with instructions to present his credentials to the new government.

Pezullo had been in and out of Nicaragua for the past month during the protracted U.S. effort to induce former president Anastasio Somoza to resign and go into exile. Throughout that period, though, he had been under orders not to offer his credentials to the Somoza government.

In opting for a friendly approach to the Sandinistas, the sources said, the admininstration is keenly aware that it is following a line that could expose it to heavy criticism from Somoza's once fermidible array of @u.s. supporters and others who fear that the Sandinistas vicotry inevitably will turn Nicaragua into a Marxist, Cuban-oriented bastion within Central America.

In fact, it is known that influential figures within the Y.S. intelligence community believe, on the basis of their analyses of the situation, that the hard-core Marxists in the regime quickly will begin trying to neutralize the influence of the junta's more moderate members and seize control.

Of particular concern to intelligence sources was the appointment of Tomas Borge, a Sandinista guerrilla leader who has espoused Marxist beliefs and who has close ties to Cuba, as interior mininster, a post that will give thim control over the police and internal security forces.

One member of the intelligence community has suggested that the Borge appointment may have been the first step in what he called a "textbook example" of gaining power "a slice at a time" by progressively maneuvering into control of key post.

Despite the misgivings of the intelligence community, reliable sources say the admininstration has chosen its conciliatory policy partly because it feels it has no choice other than to deal with the reality of the Sandinista victory and partly because of belief that the government is too new, too untested and too full of diverse elements to predict with any accuracy what ideological path it will take.

This essentially soft-line approach represents a victory for Secretary of State Cyrus R. Vance and those State Department officials who have been grappling with the Nicaragus situation on a day-to-day basis: Pezzullo, Assistant Secretary for Inter-American Affairs Viron P. Vaky and special ambassador William G. Bowdler.

But, the sources stressed, the strategy that has been threshed out during the past few days also has the general acquiesence of President Carter's national security affairs adviser, Zbigniew Brzezinski, and the National Security Council staff.

When the administration decided last month to press for Somoza's ouster, the NSC is known to have been very concerned about the consequences of a Sandinista takeover and to have sought unsuccessfully to find a way of throwing the succession to more moderate elements.

Instead, the Sandinista victory has made Nicaragua the first Latin American country to come under the control of a radical guerrilla movement since Fidel Castro began the Cuban revolution and seized power 20 years ago.

According to the sources, administration officials remember very vividly the intense controversies that were triggered in U.S. foreign plociy and domestic politics by Cuba's "going communist."

And, the sources stress, no one in the administration can give any guarantees that the same thing won't happen in Nicaragua, that won't follow the Cuban model internally and also become a base for trying to export subversion to other parts of Central America and the Caribbean.

But, the sources add, there is also the chance that it could move toward some form of social democracy that would mix elements of Marxism with western democracy.

In this respect, the sources said, U.S. officials have been encouraged by the fact that the junta and subsidiary offices in the new government contain representatives of non-Marxist political groups, of factions allied with the Catholic church, of the middle class and even of the basically conservative Nicaraguan business community.

How influential these forces will be on the actual workings of the new government is still unclear. But, the sources point out, the junta, in its dealings with the United States so far, has been generally courteous and friendly even on points of disagreements, and has scrupulosly adhered to its word on matters such as preventing reprisals against Somoza's soldiers and avoiding blooshed.

In addition, the sources noted, while the new government can be expected to turn to Cuba for help and adivce, it also owes debts to other Latin American countries - among them Mexico, Venezuela and Costa Rica - which championed its cause and which can be expected to counsel it to take a moderate, noncommunist course.

Finally, the sources said, the administration also calls that, in the view of many Latin American analysts, a more conciliatory U.S. attitude toward Castro two decades ago migh have helped to divert Cuba from communism.

For that reason, they added, important elements of the administration's strategy are to recognise that some of the things that happen in Nicaragua during the coming weeks mihgt have anti-American overtones and to resist letting them become provocations that could exacerbate tensions between the two countries.

As one source summed ti up: "Whether patience and goodwill will be enough to get our relations on a sound footing is far from clear. The junta has been in power down there for only four days, and it's going to take three, four months or more to see where they're going. Until then, anyone who professes to know with certaintly what's going to happen in Nicaragua ow with certaintly what's going to happen in Nicaragua is talking through his hat."