The United States now insists on an "accelerated pace of change" in South Africa after President Pieter W. Botha's "apparent commitment to negotiation" with his opponents, White House national security affairs adviser Robert C. McFarlane said today.

McFarlane said that Botha's speech to a National Party congress in Durban included some points anticipated by the Reagan administration but that others were "less concrete than they were expressed last week."

McFarlane had told South African officials in Vienna then that, unless changes were made in their government's system of racial segregation known as apartheid, President Reagan probably could not sustain a veto of a bill awaiting final Senate action and containing economic sanctions against South Africa.

Congressional sources said today that Botha's unspecific discussion of steps to ease apartheid will likely increase pressure on Reagan to sign the bill if it clears the Senate. Details on Page A27.

Speaking to reporters here, McFarlane repeatedly qualified his assessment of Botha's address by saying the United States would wait to see whether blacks and others in South Africa view Botha's remarks as "credible."

If it is seen that way, the speech "will work," McFarlane said. If not, "it won't."

McFarlane added that "there does appear to be something new but, whether that is found to be credible by the other side . . . one can only judge after a few days."

Later, in an interview on Cable News Network, McFarlane was more critical, saying he was disappointed that Botha did not lift South Africa's state of emergency. Referring to the rejection of Botha's statement by Anglican Bishop Desmond Tutu, McFarlane said, "That is a very telling indication, I think."

McFarlane told reporters that Botha's promise not to yield to pressure from his government's enemies or western critics does not change Reagan's view that the United States should continue its policy of "constructive engagement" with South Africa. Reagan, who did not listen to the speech while vacationing at his ranch northwest of here, was briefed on it by McFarlane.

"The president believes strongly that the U.S. policy has resulted in constructive change in the past four years . . . that we must stay engaged and use our influence to achieve constructive change in the future, and he intends to do so," McFarlane said.

McFarlane said the speech did not change Reagan's view on the sanctions legislation. "The president's views on what helps and what hurts hasn't changed," he said, adding that Reagan still thinks some provisions of the bill would hurt South African blacks.

Reagan has not decided whether to sign or veto it, McFarlane said.

An "important measure" of Botha's address, McFarlane said, "is to stand back and to say, as compared to two weeks, three, four or a month ago, is there something new or different today that didn't exist then. Whether it is a lot or a little, is it new and thus is there an apparent commitment to change?"

"There does appear something new," McFarlane said, citing Botha's "commitment to a negotiation" at several points in the speech.

"But again, it is whether or not the specifics that were included -- citizenship, the move away from the homelands policy, the ability of all to participate in shaping political decisions, the appearance of some ultimate federated system -- are really found credible or not," McFarlane said.

McFarlane refused to specify a timetable on U.S. expectations for negotiations among Botha and his opponents, but said that "in a matter of days, surely no more than weeks," the participants and agenda should be in place and "tangible evidence" produced "of the commitments implied today."

"What is at issue is an end to apartheid," McFarlane said. "It is not for outsiders to prescribe exactly how that end will come . . . . The United States can't dictate or prescribe how that process plays out."

Despite reports last week that Botha might grant jailed black nationalist leader Nelson Mandela an unconditional release or create a fourth chamber of parliament for blacks, Botha specifically rejected both moves.

McFarlane said it was "too bad" that Botha did not propose more concrete measures for sharing power with South Africa's black majority. McFarlane said that when he met in Vienna with South African officials, there was an "apparent commitment" for more political participation by blacks and others.

After today's speech was broadcast, Reagan consulted by telephone with Vice President Bush, Secretary of State George P. Shultz and McFarlane, who then read reporters a statement that he said Reagan had helped prepare.

"Apartheid is a system that has long plagued South Africa, and it must be ended," he said. "A cycle of violence and repression has engulfed South Africa. This, too, must end.

"South Africa must find peace with itself and develop a system of government which accommodates the legitimate rights and needs of the black majority and provides for justice, equality, respect for human rights, and, most importantly, government based on consent," he said.

McFarlane described Botha's statements as "important," citing the call for negotiations and his recognition of "the principle of participation and responsibility of all South Africans in their country's future."

"We look for an early implementation of those principles through a process of negotiations between the South African government and the leaders of South Africa's other communities," he said.

Botha's statements on citizenship, he added, and "these and other ideas contained in the speech must be clarified."