U.S. military leaders in private meetings are sounding advance warnings against going along with any civilian plan for a gradual increase of American forces in Central America, it was learned yesterday.

These officers, including several of four-star rank, are telling colleagues that they must protest any effort to repeat the Vietnam experience, where U.S. military forces were gradually committed without the backing of the U.S. civilian population. The military's heightened concern about Central America stems in part from the belief of some of the leaders that President Reagan sooner or later will face the choice of sending in more U.S. forces or of permitting communist-sponsored takeovers in the region.

One four-star officer, informed sources said, has suggested that the Joint Chiefs of Staff prepare a "white paper" detailing why Central America is important strategically to the United States and what would be required in U.S. troops, equipment and dollars to combat the pro-communist forces there effectively.

His idea, which the chiefs as a body has not adopted, according to its spokesman, is to prepare the study now so that everybody would know in advance what the U.S. military believes would be needed to do the job.

And if policy-makers and the public did not want to make the full commitment, the argument goes, then the U.S. military should recommend staying home.

If the White House and Congress still insisted on half-measures, the armed services would carry them out, of course, since that is their constitutional responsibility. But, under one scenario being discussed, the Joint Chiefs would formally protest a Vietnam-style gradual buildup in Central America.

While the public posture of the Joint Chiefs is that military training and economic assistance, not combat troops, are the way to combat the communist threat, some generals experienced in that region said they doubted this would be enough.

"They know it is ridiculous to pretend that 55 American military advisers are enough to do the job in El Salvador when you look at everything the Russians and Cubans are doing in the region," one senior officer said.

The Pentagon estimates that up to 8,000 Soviet civilian technicians are in Cuba, along with 2,500 military advisers and the Soviet "palace guard" brigade of 2,600 to 3,000 troops. In Nicaragua, the Pentagon estimates there are 2,000 Cuban military advisers, 70 Soviet military advisers and 5,500 Cuban civilian technicians.

However, the recent discussions showed that the military leaders' warning against a Vietnam-style commitment in Central America does not necessarily mean that they favor a major U.S. intervention. Some strongly oppose such a move. Among the opponents is Gen. John W. Vessey Jr., chairman of the Joint Chiefs.

"I want to tell you that neither I nor any member of the Joint Chiefs of Staff, nor the civilian leaders in the Department of Defense advocate introducing U.S. combat forces to try to implement an American military solution to the problems of Central America," Vessey said recently.

"We don't want that to happen," Vessey continued, "and the governments down there that are trying to get our help don't want it to happen. But military assistance and training assistance are urgently needed. Insurgencies such as those in El Salvador must be handled by a combination of political, economic, military and social efforts. We already have too many Soviet-supported communist governments in this hemisphere, and we don't need any more."

Gen. David C. Jones, Vessey's predecessor as chairman of the Joint Chiefs, expressed similar reluctance. At a White House meeting in 1981, according to a participant, Jones opposed proposals by former secretary of state Alexander M. Haig Jr. to use American forces to seal off Cuba and Nicaragua.

However, some U.S. military leaders in private discussions on Latin America are asking their comrades-in-arms what they will recommend if Reagan's present policy in Central America, featuring limited American forces, fails.

"If you stay out, here's what's going to happen," said one Army general. He then painted this picture, according to several who have heard his private assessment:

Thousands of Latin Americans will flee north to escape their communist rulers, stopping first in Mexico and then moving into the United States. The tide of refugees eventually will become too much for the American economy to absorb. Some American forces will then have to be withdrawn from Europe to seal the U.S. southern border.

Asked if fellow officers took the general's warning seriously, one military leader replied, "Nobody said he was out of his mind."