Andrew Young, American Ambassador to the United Nations, warned radical Africans and their Communist and Third World allies today that the United States would not be bludgeoned into accepting unenforceable sanctions against South Africa.

Young was speaking to the U.N.-sponsored conference for action against apartheid, which is still split between radicals and moderates fighting over a final resolution.

Buoyed by the Nigerian-American rapprochement that Young engineered last winter after years of estrangement during the Ford and Nixon administrations, the Ambassador warned:

"It would not make much sense to make agreements here that would be repudiated by our Congress and repudiated by our people."

On the eve of the new Anglo-American initiative to bring black-majority rule to Rhodesia, Young said he had sat through days of speeches here in which the Carter administration had been "condemned and blamed and blasted for its imperialism, neo-colonialism and what-have-you."

"I would only call your attention to the fact that we are probably much more condemned by the government of South Africa," he said, which recently has characterized U.S. policy as "strangulation with finesse."

"Just as I cannot accept the condemnation of this conference," Young said, "neither can I accept the condemnation - or the credit - that would go with agreeing with Prime Minister [John] Vorster and his South African government."

The key conference issue involves radical demands for imposing mandatory total economic sanctions against South Africa, a ban on either peaceful or military nuclear cooperation with it, and the cessation of all investment and loans there. None of the demands appears acceptable to the United States, South Africa's principal Western ally.

Young told the delegates from more than a hundred countries, "Our job is to move a trillion-dollar economy and a massive establishment, which can work for good or ill will, in the direction of a moral and responsible position toward the continent of Africa."

South Africa's policy of apartheid, which he likened to cancer, is "a policy of discrimination and racism that most Americans had known at home in the not-so-distant past," Young said. But he insisted that it is a disease which "can be cured and it is not necessary to kill the patient."

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Expressing hope that white South Africans will realize where their self-interest lies, Young noted "how very quickly the tides of history can switch" by invoking the example of President Carter himself:

"Ten years ago I would not have believed that a person such as Jimmy Carter could even exist in rural Georgia, much less become president of the United States."

In an apparent criticism of Communist support for violence to bring about change in Africa, Young said: "Armed struggle is advocated most vigorously by those who are thousands and thousands of miles away and whose only contribution to the struggle is the rhetoric of bitterness and frustration."

Brig. Joseph R. Garba, the Nigerian external affairs minister who was chairing the meeting, left no doubt about how Nigeria sees the importance of Young.

Hailing the American envoy as a "great Africanist," Garba said, "To Africa, Andrew Young represents a new and emerging black conscience coming out of America," a "symbol of a new and constructive policy of the United States toward Africa."

No matter what America's shortcomings, Garba insisted in reply to the radicals, the Carter administration has "done more than any of its predecessors to help in the fight against apartheid."

Young, who has indicated a belief that nothing can be done in Africa without Nigerian support, credited the Lagos government with bringing about a "new sensitivity of the West" about apartheid and the problems of southern Africa in general.

Nigeria, Africa's most populous country whose oil revenues have boosted its gross national product beyond South Africa's this year, has always exercised its power in a "statemanly, wise and restrained way," Young said.

Nigeria exercised a key influence by financing Rhodesian nationalist guerrilas. It is believed to be ready to provide troops for a U.N. force that, under the new Anglo-American plan, would keep order during the transistion from white to black rule in Rhodesia.