U.S. Backing for Talks Urged : Next 60 to 90 Days Crucial to S. Africa’s Future, Envoy Says

South Africa’s ambassador to the United States, saying the next two to three months will be crucial to his country’s future, appealed Thursday for American support for government-sponsored talks seeking a new, multiracial constitution.

The ambassador, Piet Koornhof, said the constitutional talks will succeed or fail largely on the question of whether key black leaders are willing to take part, and he suggested that the United States can help influence their decision.

“The negotiation process is on,” Koornhof said in a breakfast meeting with editors and reporters in The Times’ Washington bureau. “The question is: Who’s going to participate?

“If I could ask for any assistance it would be for . . . getting (participation in the talks) as widely representative as possible,” he said.

South African President P. W. Botha has announced an effort to seek out potential black negotiating partners for talks with the white-minority government toward a new constitution. So far, however, few black leaders have endorsed the plan.

“Not in my lifetime has there been more genuine hope than at this moment in time,” Koornhof said. “I must add that I don’t think that will remain so indefinitely. . . . The next 60 to 90 days are critical.”

He appealed to the Reagan Administration and to American newspapers and television networks “to condemn the cycle of violence” in South Africa. The Pretoria government has blamed most of the violence, including the explosion of a car bomb on Thursday that injured more than 70 in central Johannesburg, on the outlawed African National Congress.

“If the world would know for sure that America is not in favor of violence in that part of the world, they can’t help in a greater way,” Koornhof said.

“In my view, the United States has still got a very good, powerful influence in southern Africa, a respected influence, and I wish you would keep it that way,” he said.

The State Department issued a statement condemning the Johannesburg bomb as repeating “a fruitless and tragic practice of using violence to effect political change. . . . We continue to abhor violence from any source and to urge restraint.”

A State Department official said the Reagan Administration has not specifically endorsed Botha’s proposal for negotiations, however, because it falls well short of the demands of most black leaders. Black leaders, including some moderates, have called on the government to include the ANC in talks and to release imprisoned ANC leader Nelson Mandela from prison; Botha has refused both demands.

“We welcome dialogue in any form,” the official said. “If the talks succeed, we’ll be delighted. But they don’t look like they’re going to get off the ground.”

Koornhof said the negotiations could consider the issues of Mandela and the ANC’s political status after they get under way.

“If the cycle of violence can be ended, it means you’re well on the way . . . (toward) the legalization of the ANC and the release of Nelson Mandela,” he said.

But he said recent talks between white liberals from South Africa and ANC leaders in Senegal had actually made those moves “a wee bit more difficult” because the whites had failed to extract a promise from the ANC to halt terrorism.

Nevertheless, Koornhof said, the ruling National Party, which has presided over South Africa’s system of apartheid for 39 years, is now committed to “the elimination of any discrimination on the ground of color, race, cultural affiliation or allegiance.”