Washington — There seem to be few secrets left in America's secret war against Nicaragua. United States officials have all but openly admitted that the Reagan administration is supporting the antigovernment forces fighting in Nicaragua. The aim, they say, is not to overthrow the Nicaraguan government but to persuade it to stop supplying arms to Salvadorean insurgents and to get the government in Managua to negotiate a regional peace agreement.
Publication in the New York Times Thursday of a US National Security Council document on American policy toward Central America and Cuba confirmed what many people who have been studying the problem in the Congress and elsewhere already knew: The US Central Intelligence Agency is involved in a mini-war in Nicaragua and pressure on the Nicaraguan government has increased as a result of ''covert efforts.''
What is still unclear to many observers in Washington is how far the fighting in Nicaragua is likely to go. At the moment, it appears to be concentrated mostly in the northern part of the country and does not appear to come close to threatening the survival of the Nicaraguan government. At the same time, however , there are unconfirmed reports of increased antigovernment attacks on the Atlantic coast of Nicaragua.
A number of American lawmakers have expressed concern that the US-backed operations in Nicaragua may be illegal. The law prohibits American support to any group for the purpose of overthrowing the Nicaraguan government or provoking a war between Nicaragua and neighboring Honduras.
But the real concern of many in Congress goes well beyond the issue of legality. They question whether the administration knows what it is doing, whether it can really exert much control over the forces fighting inside Nicaragua, and whether the whole operation might backfire by solidifying support for the Sandinista regime.
Some in Congress also worry that American intervention in Nicaragua, however indirect, will undercut the credibility of US criticisms of Soviet involvement in Poland and Afghanistan and revive anti-Yankee feelings in much of Latin America.
The ultimate concern is that if the mini-war became a major war, the fighting might spread to Honduras, and the Sandinistas might call for help from Cuba. This might force President Reagan to consider increasing the American involvement in the conflict.
With these points in mind, members of both the Senate and House Intelligence Committees are calling for a review of CIA activities in Central America over the next few weeks. Lawmakers who are not privy to the classified information available to the oversight committees have a tendency to defer to their colleagues in these committees on such matters. But Rep. Michael D. Barnes (D) of Maryland, chairman of the House subcommittee on hemispheric affairs, will try next week to attach to a foreign aid bill a ban against all covert operations in Nicaragua.
Representative Barnes calls what is happening in Nicaragua a ''1980s version of the Bay of Pigs,'' a reference to the abortive CIA-backed invasion of Cuba in 1961. Another critic of US policy in Central America, Rep. Gerry Studds (D) of Massachusetts, calls it a ''slow-motion Bay of Pigs.''
Many on Capitol Hill are reluctant to go this far, however. Their distaste for the Sandinistas outweighs their distaste for covert operations. But it is now clear that they cannot ignore the issue. The ''secret war'' has now been the subject of numerous newspaper and magazine articles, not to speak of prime-time television reports. The New York Times says high-ranking State Department officials raised questions in White House meetings last week about the legality of the US involvement. Reporters from Newsweek magazine and the Washington Post have traveled into Nicaragua with antigov-ernment Nicaraguan troops who speak openly of the American support they are getting and of their desire to overthrow the Sandinista regime.
According to Newsweek, the top ''counterrevolutionary'' combat leaders now inside Nicaragua once served under Nicaragua's former strong man, Anastasio Somoza Debayle. But, despite their ''rather unsavory Somozan roots,'' they have won a surprising degree of local support in the northern mountains, says Newsweek. This was a region from which Somoza's national guard recruited heavily and many of the relatives of the guardsmen still live in the region.
Honduran sources are quoted as saying that the Nicaraguan Democratic Force (FDN), the largest anti-Sandinist group, has 4,000 to 5,000 men fighting inside Nicaragua. Sandinist estimates are much lower than that.
FDN leaders say that their aim for the immediate future is to seize a town, possibly the town of Jalapa, not far from Honduras. Having done this, they would then control an airstrip and might conceivably declare the area a ''liberated zone.''
The clearest and most elaborate public explanation of what the US is doing in Nicaragua came earlier this week from Jeane J. Kirkpatrick, the US ambassador to the United Nations. In an appearance on the ABC television program ''Nightline'' Tuesday, Mrs. Kirkpatrick did not directly admit that the US was backing the anti-Sandinist forces, but she came close to doing so. At the same time, she vigorously denied that the administration was breaking the law.
Mrs. Kirkpatrick started one sentence by saying: ''Whatever we're doing inside. . .'' and then stopped herself before saying ''inside Nicaragua.'' She then continued: ''Whatever support we're offering, and I don't say we're offering any, but if we are offering support to anyone in Nicaragua, it's with the permission of the Congress. . . .''
She said: ''If you have economic pressure and it fails, and you have moral pressure and it fails, and you have political pressure and it fails, because you've got a repressive regime that's progressively totalitarian in character, then in fact the United States might conceivably decide that it would like to enhance the pressure on the government of Nicaragua.'' The purpose, she said, would be to persuade the Nicaraguan government to negotiate a peace in the region and ''cease fomenting and making civil war against its neighbors.''