Secretary of State Alexander M. Haig Jr., in the latest of a series of blunt statements about Central America, expressed "grave concern" yesterday about a totalitarian trend in Nicaragua and declared that El Salvador will follow suit unless the United States provides increasing economic and military support.

Haig, speaking in Palm Beach, Fla., also declared that Washington has "proof positive" that Cuba is "heavily engaged" in the supply of contraband weapons to Central American insurgencies. The secretary of state charged that Cuba is serving as "a proxy for the Soviet Union" in a step-up of Western Hemispheric activities.

As have other recent statements, Haig's remarks raised the possibility that the United States is prepared to go beyond diplomacy and foreign aid to the use of military means.

But the secretary's comments, like the threats last February to "go to the source" of Central American troubles, left unclear whether the administration is serious about the use of force or whether it is engaging in a psychological "chicken game" aimed at scaring the Cubans and Nicaraguans away from their support of leftist guerrillas.

In either case, the administration plainly wants to keep its real intentions secret for now. But interviews with a variety of sources familiar with Washington's internal decision-making process indicate that, while various military contingencies are under study, there are no immediate plans to put any of them into operation.

The sources, some of them senior administration officials, said that, to the best of their knowledge, President Reagan has not yet decided what new actions to take in the Caribbean region and is inclined toward a go-slow approach. They said Reagan was trying to signal that attitude when he told a news conference last week: "We have no plans for putting Americans in combat any place in the world . . . ."

That, the sources stressed, doesn't mean that the administration, which has staked considerable prestige on backing the civilian-military junta in El Salvador, is not worried about the government's apparent inability to put down the guerrilla challenge. The sources added that U.S. officials ascribe much of the blame for the military stalemate to the guerrillas' continued ability to receive arms smuggled to them from Cuba through neighboring Nicaragua, whose revolutionary Sandinista government is regarded by the administration as moving increasingly toward communism.

In Managua, Nicaraguan Foreign Minister Miguel d'Escoto yesterday announced the resignation of former junta member Arturo Cruz as Nicaragua's ambassador to the United States.

Cruz said that when he had accepted the post he had agreed to serve only six months. "Nine months have already passed and at this time when relations with the United States are difficult, I have decided to retire because I do not have the means to be able to help normalize these relations," he said.

Sources said that cutting off the flow of arms into El Salvador from the outside has reemerged as a high-priority concern of the administration. That concern, in turn, has spawned a spate of press reports about the administration examining such courses as a naval blockade of Nicaragua, the staging of large-scale air and sea maneuvers near Cuba, a new attempt at a trade quarantine of Cuba or even invasion or subversion campaigns against the two countries.

Publicly, no U.S. official has acknowledged that any of these ideas is under consideration. However, Haig has played a big part in keeping the speculation at a high pitch, first through a series of interviews in which he hinted that the reports are true and then in public forums, notably an appearance Thursday before the House Foreign Affairs Committee, which he appeared to be using as a forum to lob warning shots toward Cuba and Nicaragua.

In his congressional testimony, the secretary denounced the "totalitarian" nature of the Cuban and Nicaraguan regimes, warned that the United States will not countenance Soviet-supported Cuban adventurism in the Western Hemisphere and refused repeatedly to give any assurances that the United States will not abet any attempts to overthrow the Nicaraguan government.

While Haig's public performance Thursday left a lot of seemingly ominious smoke in the air, other officials were privately less threatening. On the same day, one member of Congress is known to have been told by high-ranking administration officials that, while studies of the military options continue, no decisions have been made and none are expected soon.

That reiterated what most administration officials have said in private since reports about military contingency planning began circulating late last summer. Such planning, described as the drawing up of option papers, has been going on sporadically since February when the administration first decided to use El Salvador as a test of its determination to "draw the line" against communist subversion.

At that time, Haig and other top officials talked, first privately and then publicly, about how Cuba's support of the Salvadoran rebels might force the United States "to deal with this matter at its source."

That rhetoric soon subsided because of the largely negative reaction it produced in American and foreign public opinion. In addition, administration sources have conceded, it was due to a lack of enthusiasm in the Defense Department for ideas such as a naval blockade of Cuba or Nicaragua.

The Navy concluded that such an operation would cost hundreds of millions of dollars and tie up so many ships that naval operations elsewhere would be crippled. In addition, Pentagon planners convinced Defense Secretary Caspar W. Weinberger and the Joint Chiefs of Staff that overt U.S. military actions in the Caribbean might provoke a countering Soviet move in another part of the world and cause a Vietnam-type backlash from the American public.

According to the sources, planning on military ways to influence the Salvadoran conflict from the outside was almost dormant until late August when Haig and his top advisers on Latin America became concerned that the Salvadoran armed forces had been forced into a stalemate that could lead to an eventual guerrilla overthrow of the junta.

At that time, the sources said, Haig asked the Pentagon to put its examination of military options back into high gear. But, the sources added, the Defense Department's doubts remain an obstacle to achieving an administration consensus on military measures.

Nonetheless, the sources pointed out, keeping alive the idea that the United States might resort to such action has a potentially potent psychological impact; and, they added, that apparently is why Haig is so anxious to keep reminding Cuba and Nicaragua that Washington retains this shot in its locker.

Following Haig's "go to the source" warnings in February, there was a temporary slowing down of the arms flow into El Salvador, and U.S. officials now are watching closely to see whether the secretary's renewed war of nerves will have a similar effect.

Even if that happens, the net effect is unlikely to be more than buying a little time while Washington waits to see if the Salvadoran junta can get an upper hand.

If the junta proves unequal to the task, the administration likely will face a difficult choice of trying to swallow a "political solution" of pressuring the junta to come to terms with the guerrillas by giving them a share of power or taking another hard look at the U.S. military options.

Since Reagan is so strongly committed ideologically to checking the spread of communism in the hemisphere, many of the sources believe that under those circumstances he would choose the military route, however difficult and unattractive.

"We're still quite a ways from that crossroads, and maybe we'll be lucky enough to avoid it completely," one senior official said. "But if the need seems clear, I don't think he'd shy away from taking action, even it means shooting."