A number of high-level U.S. diplomats in Central America say they are increasingly concerned about what they call the "politicization" of the foreign service and denigration of its role and objectivity as the Reagan administration has juggled personnel and toughened policy in the region.

In recent conversations around the isthmus, American diplomats have privately criticized the ascendancy of what one called "foreign policy amateurs" in the administration and their sniping at the State Department and its diplomats in the field.

Several of the diplomats questioned administration rhetoric and policy decisions in relation to their own reporting. But most said their complaints are based less on objections to the direction of policy than on what U.S. Ambassador to Costa Rica Francis J. McNeil described as problems of "personalities" and seeming threats to the studied professionalism of the foreign service.

McNeil, the only ambassador appointed by former president Jimmy Carter in the region to be retained any significant length of time by President Reagan, is slated to leave his post here this month. Of a range of diplomats interviewed in several Central American countries, he was the only one willing to speak out publicly on the subject.

In an interview at the embassy Friday, McNeil denounced "attacks on the foreign service" by "the leakers" in Washington. He spoke in reference to news reports surrounding the announcement two weeks ago that Assistant Secretary of State Thomas O. Enders and Ambassador to El Salvador Deane R. Hinton were to be replaced and U.S. policy toughened.

In those reports, senior administration officials were quoted as describing Reagan's "unhappiness with the execution of U.S. policy" in Central America and the president's desire to fill jobs with people of unswerving loyalty.

"In the American foreign service a lot of people serve at some risk," McNeil said. "It doesn't do us or the nation much good to be labeling them as 'cookie pushers.' "

A U.S. official said May 27 in Washington, "You don't handle Central America politicies with tea and crumpets on the diplomatic circuit."

In a separate interview with the Dallas Times Herald earlier in the week, he said, "To suddenly find out that senior people in the White House are charging the foreign service with being soft on communism--that's McCarthyism."

The way that Hinton was relieved of his difficult post amid speculation by anonymous administration officials that he was "burned out" caused particular resentment among professionals in the region.

"Somebody threw Hinton in the bag there," McNeil said. "I don't think we understand the importance of the foreign service and the sacrifices that some people like Deane Hinton have made."

Some foreign service officers have criticized the current administration for removing a number of career diplomats from their areas of specialty because of suspect loyalty. McNeil criticized both the Carter and the Reagan administrations for "the notion that service to the previous administration per se meant that you were unreliable," and the way "the foreign service came to be perceived as the instrument of the previous administration."

Another career diplomat in the region noted "the way the last two administrations polarized the foreign service politically," and echoed a number of his colleagues' talk about the growing number of officers who "see themselves explicitly as Republicans or Democrats."

Some career officers stationed in Central America appear to have adapted well to the situation.

Ambassador John D. Negroponte in Honduras, a professional foreign service officer, has come to be closely identified not only with the hard line of the Reagan administration policy supporting the covert war against Nicaragua, but also with right-wing political forces in Congress.

Negroponte is known among other U.S. diplomats on the isthmus for his practice of criticizing their reporting when it does not match his. One of his colleagues recently called him "the War Party's" man in Central America.

In the deluge of leaks out of the White House last month about changes in the administration's Latin America team, there was no mention of the possibility of Negroponte's removal.

Many other members of the foreign service, however, prefer to play more rigorously the role of disinterested public servants. The epitome of this type, in the eyes of many, was Hinton.

According to one diplomat, although the corps is aware that the elected administration is charged with making policy, and the career envoy's role is primarily to provide "grist for the mill" from the field, "the people who know the most" about a given country are often "the farthest from power."

McNeil was one of the architects of the "two-track" plan for El Salvador proposed earlier this year by Enders. Its objective was to continue strong support for the Salvadoran government and military while appearing to open the door to negotiations with the left. This is a line that even many Salvadoran Army officers have suggested because of its potential for weakening the propaganda of the left and contributing to division in the guerrilla ranks.

But the president's national security adviser William Clark and U.N. Ambassador Jeane J. Kirkpatrick, who have no foreign service experience but have come to dominate major policy decisions on the area, reportedly relieved Enders because they thought the very mention of negotiations in El Salvador appeared a sign of weakness.

McNeil said that even though he was considered at one point for the position of special ambassador to Central America--his name having been proferred by Enders--he told Washington that in such an environment someone was needed with more "political clout."

The man who got the job was former senator Richard Stone, a conservative Democrat remembered by many Nicaraguan officials as having personally antagonized them when they visited Capitol Hill in 1979.

When Reagan took office, virtually all the experts on Latin America in the part of the State Department concerned with the region were removed. But they were replaced by diplomats with a generally recognized high level of competence. Now some foreign service officers are concerned that the trend is toward the replacement of professionals with more ideologically committed "amateurs."

This does not appear to be an automatic reaction, however. There is little criticism from diplomats in this region, for instance, of the choices to replace Enders and Hinton, even though the new assistant secretary, Langhorne A. (Tony) Motley, is not a professional diplomat.

"I think all of us who know something about foreign affairs have some problems with political appointments. The wider the practice the more you get turkeys," said one professional. But he added, "I think Motley is an outstanding political appointment."

But there is concern that the respected appointees finally named by Secretary of State George P. Shultz may only have been found as an afterthought when their predecessors had already been axed publicly by the White House.

One foreign service officer here, attempting to put the most benign possible face on the recent personnel changes, said that Clark and Kirkpatrick simply and understandably wanted to have people they know and feel comfortable with in key positions. Then the officer added absently, "They don't seem to know many people in the foreign service."

McNeil's replacement, Curtin Winsor Jr., is described in an embassy press release as the president of Winsor Pittman Coal Co. in West Virginia, a member of the Reagan transition team and a foreign policy adviser during the campaign. He spent four years in the foreign service before resigning in 1971 to be special assistant to the chairman of the National Committee of the Republican Party.