Four years ago the Reagan administration reversed what it regarded as an overly fastidious human-rights decision by President Jimmy Carter's State Department and began easing sanctions against the rightist government of Chile.

Last week that reversal became a source of uneasiness, if not embarrassment, as the Chilean governmentlaunched a new crackdown on dissidents that included a roundup of 5,000 men who were herded from a slum into a Santiago soccer stadium.

The depth of U.S. disenchantment and frustration over the course taken by Chilean President Augusto Pinochet was revealed by Secretary of State George P. Shultz at a meeting of hemispheric foreign ministers in Brasilia.

When asked about Pinochet's declaration of a state of siege and the subsequent wave of arrests in Chile, he said: "I love Chile and its people, its vibrancy, and it is sad and disappointing to see the developments there . . . . They are very disappointing to us, and I'm sure that Gen. Pinochet is well aware of that fact."

Shultz's remark was reinforced by a State Department statement questioning whether leftist-inspired terrorism "is of such dimensions as to justify the extreme measures associated with a state of siege." The department also announced it was "reviewing carefully how recent developments may affect U.S. interests."

U.S. officials said privately that this "review" should not be regarded as a signal of a drastic shift in policy. They contended that the United States, lacking major influence or leverage in Chile, can only try to persuade and to appeal for moderation.

But the officials made clear their concern that Pinochet's stubborn hard line could exacerbate the conflict between the extreme left and right and make Chile a cockpit for the same ideological conflicts that the United States is trying to arrest in Central America.

The situation dates to 1973 when the Chilean military overthrew the avowedly Marxist, Communist-supported government of Salvador Allende in a bloody coup and instituted one of the most repressive dictatorships in the world. Pinochet's brutal tactics reached to Washington, where a former Chilean diplomat, Orlando Letelier, and an American, Ronni Moffit, died in a 1976 car bombing. Chilean intelligence officials subsequently were found to have had a part in the bombing.

Chile became the most conspicuous target of Carter's activist human-rights policy, with the two countries barely on speaking terms. That situation changed dramatically when President Reagan took office in 1981 and immediately began lifting the various sanctions.

The new approach was spelled out in March 1981, when John A. Bushnell, then acting assistant secretary of state for inter-American affairs, told Congress that the Reagan administration would not tolerate a human-rights "double standard" that served only to damage U.S. security and commerical interests. Bushnell contended that Chile was making important advances in easing its dictatorial rule.

But the Pinochet government continued to crack down hard at the first signs of internal dissent. It also announced its intention to keep power until 1989 and then, using a carefully contrived plebiscite system, transfer it to a junta-approved candidate who would serve as president until 1997.

The lack of progress toward democratization was so embarrassingly obvious that 14 months ago the administration began calling for "dialogue" between the Pinochet government and "democratic forces" leading to a democratically elected civilian government.

Last April the administration even took the unusual step of trying to spell out its new approach to Chile in a speech at the University of Arkansas by James H. Michel, a deputy assistant secretary of state.

The speech centered on Michel's contention that Chile's problems stemmed from the failure of its people "to find and institutionalize a durable consensus of the center and to diminish the extremes of left and right."

In short, U.S. officials said last week, their concern is that continued rightist rule throughout the time envisioned by Pinochet will provide fertile ground for leftist extremists to incite terrorism and perhaps civil war.

While the United States has become increasingly insistent that a dialogue leading to a centrist solution offers the best hope of averting extremism, its calls have fallen on deaf ears. U.S. officials lay part of the blame on the failure of Chile's old political parties, from the nationalistic right to left-leaning Christian Democrats and Socialists, to settle on a coherent program.

But, officials acknowledge, the biggest problem remains Pinochet's insistence on sticking to his course and his skill at keeping the military united behind him.

The officials also contend that the United States has not been able to exert much influence with the Chilean armed forces, largely because of the military's resentment of a 1981 congressional act barring U.S. military aid to Chile unless the president certifies, among other things, that Chile has cooperated with the Letelier murder investigation. The Pinochet government's obstructive tactics have made such certification impossible, and it generally is agreed that congressional liberals can block any attempts to lift the certification requirement.

As a result, the administration has been left in the position of watching the situation with increasing nervousness and issuing statements about how it would like the Chileans somehow to come together and work out a transition to democracy.