Presidential national security affairs adviser Robert C. McFarlane bluntly warned South African officials that President Reagan would not be able to sustain a veto of legislation imposing economic sanctions on South Africa unless there is an "accelerated movement" away from apartheid, a senior administration official said yesterday.

He said McFarlane gave this assessment and called for changes in South African policy at a candid meeting in Vienna on Thursday with a delegation of high-ranking South African officials headed by Foreign Minister Roelof (Pik) Botha.

"I say that they the South Africans came away from that meeting with a clear sense that they have missed some of the factors shaping opinion in this country and elsewhere and are under no illusions that the status quo can be supported," said the U.S. official, who spoke on condition that he not be identified.

He said McFarlane emerged from the meeting convinced that the South Africans are "very conscious that they must make significant changes to respond to black wishes for expression and influence on those who govern them . . .."

Both chambers of Congress have overwhelmingly passed legislation imposing significant economic sanctions on South Africa, but final Senate action on a conference report is not scheduled until Congress reconvenes in September. Reagan has opposed sanctions in the past but has not said whether he will veto the bill.

McFarlane told the South Africans that the president will base his decision on what the white minority government does to improve conditions for blacks, the official said.

In the wake of the Thursday meeting and a follow-up session the next day between Botha and Assistant Secretary of State Chester A. Crocker, U.S. officials expressed optimism that Prime Minister Pieter W. Botha will announce major policy changes Thursday when he addresses a regional conference of the ruling National Party.

But the senior official who briefed reporters yesterday said he thought it unlikely that the South African government would go as far in abolishing the most onerous restrictions of apartheid, the policy of racial separation, as published reports have indicated.

United Press International, quoting congressional sources, said Friday that South Africa was considering lifting the state of emergency, easing passport laws that restrict the movements of blacks, and freeing black leader Nelson Mandela, who has been imprisoned for 22 years.

U.S. News and World Report, in an article prepared for its Monday issue, quotes South African sources as saying that Prime Minister Botha is considering proposals to offer blacks a role in a central government; declare South Africa a single, unitary state that could result in the scrapping of tribal homelands; grant common citizenship to all 30 million South African citizens, and hold a national meeting among white, black, Indian and mixed-race leaders to agree on a new power-sharing constitutional structure.

U.S. officials said that these and other options were discussed in the Vienna meetings but that U.S. representatives pressed for a dialogue with black leaders rather than insisting on any specific proposal.

Throughout his presidency Reagan has taken a friendly approach to the South African government and advocated a policy of "constructive engagement" under which South Africa has been pressed privately to make changes but has not been publicly pressured by the U.S. government. Reagan has said sanctions that would force U.S. businesses to withdraw from South Africa or limit investments there would be harmful to blacks. McFarlane reaffirmed the president's support of constructive engagement and also expressed his reservations about sanctions, the official said. But the national security affairs adviser then went on to express what he called "a personal view" that "the emotional climate" in the United States would not make it possible for Reagan to sustain a veto unless South Africa makes fundamental changes involving power-sharing with blacks.

The U.S. official said the initial South African response was to warn of growing Soviet influences in southwest Africa. McFarlane said that the Reagan administration shared this concern, but that most Americans were far more concerned with what they perceived as the "bloody-minded" attitude of the South African government toward blacks.

McFarlane was described as saying that "the American people and the Congress have a relatively superficial view of the nature of the problem in South Africa but they do see that there are legitimate black grievances and that in some cases they associate the history of black repression with the racial turmoil in this country. And they say we were able to solve this by taking certain actions, they ought to be able to do this in South Africa."

The U.S. strategy, as described by administration officials, was to use the pending sanctions legislation as leverage to promote changes in South Africa that might ultimately avert the sanctions. One official said that if South Africa made genuine changes that were accepted by black leaders, Congress might not impose the sanctions or, alternatively, that Reagan might be able to sustain a veto under these circumstances.

Leading Republicans who previously opposed sanctions supported strong measures against the South African government this year. Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) reportedly has urged Reagan to sign sanctions legislation, which was put over until September to avoid a threatened filibuster.