J.H.A. Beukes, South Africa's ambassador to the United States, expressed doubt yesterday about whether sending a special U.S. envoy to his country would be useful unless the purpose is to improve relations between the governments.

"The only role that I see for such a person would be to restore some form of confidence and credibility in the channels but not with a particular message to achieve anything of political substance," Beukes said in an interview.

"That is the kind of thing South Africans, black and white, will do inside the country," he added.

On Thursday, White House spokesman Larry Speakes said that sending someone such as Sen. Paul Laxalt (R-Nev.), who served as a special presidential envoy to then-president of the Philippines Ferdinand Marcos, had been discussed at the White House but that no decision had been reached.

Yesterday he said the proposal, which he associated with Sen. Nancy Landon Kassebaum (R-Kan.), chairman of the Senate Foreign Relations Committee's African affairs subcommittee, had been "a bit misunderstood."

While the idea had been "noted" in the White House, he said, "there are no plans to do that."

Beukes cast doubt on the Reagan administration's ability to play a role in promoting negotiations between white and black South African leaders. He said that would depend partly on its "access to" and "standing with" the black nationalist African National Congress.

"You can only play a role of honest broker if you have influence with all parties," he said. "I just don't know if they the Americans have it." But, he added, "at the moment there is not the greatest of U.S. influence in South Africa, either."

Asked about U.S.-South Africa relations, Beukes replied, "Let's just say they have been better before."

Despite the recent failure of a mediation effort by British Commonwealth nations, Beukes said, his government remains willing to include the ANC as one of several black groups in any negotiations provided it agrees to democratic and nonviolent policies. But he said the Commonwealth failed to obtain such a commitment from the ANC.

"The ANC is not committed to a nonviolent negotiating process and wants to be the only representative of black South Africa," he said. "There is no way the South African government can accept that."