The Reagan administration acknowledges that "the past 12 months have seen a depressing cycle of deterioriation" in efforts to change South Africa's system of strict racial segregation. But, while granting that much of the problem should be blamed on a "bloody-minded" South African government that has a "tendency to shoot itself in the foot," the administration also believes that "Americans are deluding themselves if they think they have decisive influence" to end apartheid through sanctions and other pressures.

In an interview with The Washington Post, a senior administration official said that this assessment of the situation explains President Reagan's determination to stick with his controversial "constructive engagement" policy of trying to influence the South African government through dialogue and persuasion, despite growing sentiment that the policy has failed and should be replaced with a tougher, more punitive approach.

At the same time, the official indicated that the Reagan administration has continued to pressure the South African government, both by urging the unconditional release from prison of black nationalist leader Nelson Mandela and by declaring its support for the elimination of all forms of legal oppression of blacks and, eventually, for a democratic, multiracial South African state.

This official, who is closely involved with the conduct of U.S. policy in Africa, declined to be identified on the grounds that, if named, his comments might be regarded as personal opinions rather than a statement of a unified administration position. He specified that the interview was intended to clarify the administration's thinking and to correct what it regards as misunderstandings and misrepresentations about the nature of constructive engagement.

Responding to charges that the administration's five-year adherence to its policy has failed to produce significant reform or to stave off the threat of racial civil war in South Africa, the official said:

"The past 12 months have seen a depressing cycle of violence. The debate we are having in this country has been fueled by the wave of violence and repressions, detentions and more violence . . . . The reform process has suffered as a result, and so has our relationship with South Africa.

"Today South Africa is less responsive and more rattled and more bloody-minded, you might say, than it was before all this began. That's due both to black disappointments and the resulting unrest on their domestic scene, and it's due also to the rising emotional American debate about sanctions and tougher action . . . .

"The rise in this country of the 'Free South Africa' movement has helped raise the issue to the forefront of American political rhetoric, and it has caused negative feedback from South Africa. I'm not saying that Americans are causing the unrest . I'm just saying that the two societies are not on the same wavelength. The clocks of the two societies are not synchronized."

The official acknowledged that the administration had been led to expect that South African President Pieter W. Botha's speech Aug. 15 would contain a blueprint for specific reform proposals by the white-minority government. Instead, Botha's speech was widely regarded as a defiant warning that Pretoria will not depart significantly from its system of white-minority rule.

"There's not much to be optimistic about right this week," the official said in reference to Botha's speech. "The government there has managed to shoot itself in the foot on quite a number of occasions recently. And there seems to be a sense of digging themselves in deeper in recent weeks."

But the official said it is premature and unwise to read what Botha said in strictly negative terms and to ignore its hints of willingess to negotiate, however vaguely phrased. He said:

"If the speech that was given on the 15th means what Pik Botha South African Foreign Minister Roelof F. Botha, who is not related to the president and others have said since then, it means that it is an important speech. Pik Botha and other officials have said that President Botha's speech should be read as an offer to negotiate. But given the fact that the two messages come across rather differently, what is one supposed to conclude? What are blacks in South Africa supposed to conclude? The opaque character of the speech has to be clarified."

The official said Reagan was misunderstood when he said in a recent broadcast interview that Botha's government was a "reformist administration" because it has eliminated segregation in some public places.

The official said Reagan was referring to what experts on South Africa call the manifestations of "petty apartheid" such as segregated hotels and swimming pools and was trying to praise the Botha government's moves ending some of those practices. But, the official insisted that, while such measures are considered encouraging, Reagan regards them only as partial, interim steps toward the longstanding U.S. goal of a dismantling of the larger, so-called "grand apartheid" system that is the codified basis of South African life and its replacement by a democracy open to all races.

The administration, while continuing to regard sanctions as "self-destructive and counterproductive," believes that the legislation expected to emerge from Congress next month is intended as a "sharp signal to Pretoria" of growing U.S. impatience at the lack of reform and is not an attempt to force U.S. policy onto a punitive course at this time, the official said.

Asked if Reagan would veto the bill if it is passed by Congress, the official said: "It's a mixed picture. It contains some positive things; it contains some negative things. It faces the president obviously with a difficult decision which only he can make. He hasn't made it yet."

The administration has told the Botha government "repeatedly and unequivocably" that it favors the "unconditional release" of Mandela from prison, the official said. In the past, South African authorities have said Mandela first must renounce violence and in effect renounce his South African citizenship by accepting citizenship in one of the black "homelands" established under Pretoria's policy of denationalizing blacks by creating what it contends are separate black states.

In his Aug. 15 speech, Botha reiterated that Mandela must renounce violence. But the senior U.S. official said, "If the name of the game is to release influential black leaders and engage them in a dialogue, you can't start out by laying down preconditions on the white side. The natural instinct of the black side will be to lay down their own conditions, and before you know it, there will be an escalation of preconditions that blocks meaningful negotiation."

Those who criticize Reagan's continued adherence to constructive engagement do not have "a viable alternative policy" or do not understand what the administration is trying to accomplish, the official charged.

"The policy is not one of quiet diplomacy: that's a tactic, not a policy," he said. "Nor is the policy one of 'friendship,' or, as some have charged, all carrot and no stick. The problem is how to be most effective -- how to use your diplomacy in the way that you think will be most effective."

"We hope to see the cycle of polarized violence and distrust ended," the official said. "We hope to find a kernel of good will that can be captured so people can find a way out of the boxes into which they've painted themselves. We hope to see genuine negotiation and bargaining, whether it's publicly visible or not.

"We hope to see the South African government take the wraps off what we sense to be an interesting package of ideas that might have something in it. I'm not saying it does. We don't know yet. What they must do is clarify what they mean."

On the other side, he continued, "we would obviously hope to see significant black leaders test the government's readiness and not simply escalate preconditions on their side. Right now, both sides are playing a procedural game, and it's time to get on with it -- to see people released, to see the state of emergency ended, to move to dialogue and negotiation."

"One of the things that makes it so hard to conduct a rational discussion in this country on this issue right now is that it is so emotional," the official said. "Sure it's emotional. I understand that. But the emotion is based on a kind of naive lack of recognition of where things were 10 years ago, where things were 150 years ago. The system has been going on there for 330 years.

"But now we have a kind of sense that, gee, there's a series of buttons on my telephone that we could push if only our heart was in the right place, and we could make the whole thing end. In other words, there is the illusion of influence. We need a greater sense of history. We also need a greater recognition of the limits on our influence in this country."

"While the violence in South Africa has maybe stepped up the pressure and timetable for change, I don't expect to see a solution in the coming months," he warned. "This is a drawn-out, protracted struggle between a white nationalism and a black nationalism. It's been going on for a generation, and it's going to go on for a while longer. But what has to be stopped -- and where we hope the limited influence that we do have can be of help -- is the deterioration that breaks down the channels of communication between the two sides."

"What would satisfy us? The answer is simple and unequivocal: an end to apartheid and its replacement by a system based on justice. We've said it repeatedly. We've never argued that simply desegregating swimming pools or park benches is an answer."

The Reagan administration, he said, seeks "the end to a system of legally entrenched racism, a system that excludes blacks from the mainstream of South African life, that defines and entrenches by law in racial terms where blacks can live and go to school, that ultimately comes down to whether they can have a stake in the politics and economics of their country.

" . . . The larger system of grand apartheid cannot work. We will not go along with it and such features as the homelands policy. We have told them they can't denationalize blacks. They can't base a settlement on giving 13 percent of the land to 72 percent of the people. We're not trying to define in specific terms what their system should be. That's for South Africans to decide. But we have made clear our belief that they should have a government that is truly based on the consent of the governed."

The official said it is incorrect to assume that when President Botha and other Afrikaner leaders say they will never accept the principle of one man, one vote, they are talking about permanent disenfranchisement of the black majority.

"To a certain extent, what they are attacking is the Westminster model, the British model that was imposed on the Dutch-descended Afrikaner whites when the British beat them in the Boer War. They see it as a winner-take-all system that would leave no place for the whites.

"The fact that many Afrikaners believe there are other fair and equitable models that could be adopted is overlooked too often in this country. It needs to be pointed out that there is a bias in this country which is inclined to say that if an Afrikaner, if a Boer did it, it must be wrong. That's a theme one finds frequently in the English-language press in South Africa, where they're still fighting the Boer War, and that theme of 'Never trust an Afrikaner' too often becomes the principal source of wisdom about South Africa in this country."

"I feel very strongly that we are on the correct course," the official said. "I would go further and say that we have not heard from any of the participants in our American debate about what would be a workable alternative. If the people say we should ban krugerrands or if the Congress were to override a presidential veto, what would we do as the next step? I would argue that our policy would continue, because when people think about it, there is no alternative other than to try to be constructively involved on behalf of those objectives that Americans share."