Zimbabwean Prime Minister Robert Mugabe, shifting his public assessment of the Reagan administration, sharply criticized U.S. policy in southern Africa today. He said it has encouraged South Africa "to become more aggressive" toward neighboring black-ruled nations.

The original hopes he had that President Reagan's policies "would lead to transformation" in Namibia and South Africa, he said in an interview, "were dashed" and instead "South Africa has tended to be more intransigent, relying on what it believes to be United States support."

Mugabe's criticism of Reagan's policy of "constructive engagement" with South Africa is likely to have repercussions beyond Zimbabwe's borders. A succession of top administration leaders, including Vice President Bush and Chester A. Crocker, assistant secretary of state for African affairs, has visited Harare to consult with him on policy toward South Africa.

Mugabe, regarded as one of Africa's most influential leaders, seems to be setting the stage for his first official visit to Washington next month.

He previously had tempered his public remarks regarding the U.S. policy of seeking better relations with white-ruled South Africa to gain concessions for the powerless black majority. In an interview two years ago, he said that he was optimistic that Reagan would be more effective than Jimmy Carter in pressing for political change in southern Africa.

Today, however, with the six-year-old Namibian negotiations apparently stalemated and South Africa increasingly charged with destabilizing its neighbors, Mugabe declared:

"The policy of constructive engagement has had the effect of encouraging South Africa to continue along the same old path of resistance to the wishes of the majority of the people in Namibia and South Africa, and, in fact, to become more aggressive against the front-line states," he said. "Almost every neighbor has something to complain about South Africa's acts of aggression, and these seem to be increasing all the time."

He described "constructive engagement" as "a policy of acquiescence in South Africa's policy."

Mugabe, 58, attacked the U.S. proposal that Namibia's independence be linked to a withdrawal of more than 30,000 Cuban troops from neighboring Angola. The United States, he said, was "using the Namibian situation to achieve an end in our region which they were not able to achieve otherwise," a reduction of the Soviet sphere of influence.

A Namibian solution is "mainly in the hands of the United States because South Africa has the greatest confidence in the United States," he said.

Mugabe also commented on a domestic issue with international implications, the possibility of again jailing six Air Force officers if they are acquitted later this month of charges that they assisted in sabotaging a quarter of the country's war planes last year.

Their trial drew international attention because the officers, three of whom have dual British nationality, alleged that their confessions were obtained through torture--which would invalidate the statement.

The prime minister referred to acquittals based on such charges as "technicalities which our judicial system inherited from western legal systems which are really not logical . . . but I suppose are humanitarian. These have the effect of getting people acquitted when in fact they are guilty."

Giving a hypothetical example, he made specific reference to the Air Force officers, but said no decision has been made yet on what to do if the men are acquitted. Many prisoners are being detained without trial, or after acquittals, under broad emergency powers.

Sitting at a desk in his sparsely decorated parliamentary office, Mugabe responded in a soft voice to questions about hundreds of atrocities allegedly committed by the Army earlier this year against civilians in southwestern Matabeleland, the stronghold of opposition leader Joshua Nkomo, in an effort to eliminate antigovernment elements. His tight-lipped expression, however, betrayed the emotions that flared several months ago when he vehemently attacked reporters, church leaders and private relief agencies that publicized the charges.

A military investigation he ordered five months ago has yet to be completed, Mugabe said. He did not say when it would be finished, but he added that anyone who committed brutalities "will be brought to book." Some relief officials who reported alleged atrocities earlier this year told reporters recently in Matabeleland that no government officials had questioned them in regard to the investigation.

"There are bound to be a few cases of overzealousness," Mugabe said. But he maintained that the actions of the Army were necessary to defend the country and had reduced banditry. In the past year, about 150 people were killed by dissidents who also were responsible for the kidnaping of six foreign tourists, including two Americans. The victims have not been seen since their abduction 13 months ago.

"We will use force against force," Mugabe said, to deal with "those who are bent on acts of lawlessness to overthrow the government."

"We tried our best" to accommodate Nkomo by bringing him into the government three years ago, the prime minister said. "One really wonders what more we are expected to do."

Nkomo has charged that Mugabe's guerrillas intimidated voters in the elections that ended a seven-year war for black rule. The prime minister fired him from the Cabinet last year on charges that he stashed arms to overthrow the government. Nkomo fled the country in March, saying the Army sought to kill him, and he only returned this week. Mugabe's government has denied Nkomo's charge.

"We cannot view Nkomo as a person entitled to be given a place in government purely because he is Nkomo," Mugabe said. "The choice of the people has got to be respected. I'm not going to give way to satisfy Nkomo's own ambitions. Nkomo has got to adjust to defeat and that is that."

He reiterated his vows to make the country a one-party socialist state but emphasized that the process should be gradual. The one-party system, he said, was "better than the amorphous situation you get in Europe," adding that "it makes for greater democracy."

Sounding like a university professor, he called socialism "the only morally tenable philosophy" because it is selfless and preaches equality. Capitalism and socialism, he said, "are opponents in the ideological sense. There is room for both of us. There should be coexistence."