Andrew Young, the sometimes controversial U.S. Ambassador to the United Nations, has made "a very important contribution to the work of the U.N." and is "highly respected" there, the organization's secretary-general, Kurt Waldheim, said yesterday.

Speaking on "Issues and Answers" (ABC, WJLA), Waldheim said Young has brought a "completely new approach" to the United Nations which has resulted in "a new confidence in the sincerity of the United States' efforts.

"I can tell you from my experience . . . that he is highly respected, especially in Africa."

The secretary-general offered pessimistic forecasts for two international points of tension, the Middle East and Southern Africa.

Unless there is a breakthrough in peace negotiations in the Middle East this year, he said, "the situation next year will be deteriorating dramatically."

But he held out small hope for a breakthrough soon, noting that the parties are still far apart. He said debate on the Middle East in the U.N. General Assembly, scheduled to begin this week, would be "rather explosvie."

While expressing appreciation for American efforts to move both sides in the Middle East toward a settlement, he suggested that a U.N. initiative might have a better chance of success. Without giving specifics, Waldheim said he "wouldn't be surprised" if the United Nations were to assume a more active role in the negotiations.

The secretary-general said the situation in Rhodesia and Southern Africa is "very grim, very dangerous, explosive."

He said he is "hoping that progress will be made" in Rhodesia toward a transfer of governmental power to that nation's black majority.

If the government of Rhodesian Prime Minister Ian Smith - which Waldheim labeled "illegal" - does not accept the latest British plan for transition, the United States should get tougher with Smith, he said.

Speaking of Rhodesia and South Africa, Waldheim said, "We have to expect strong action by the General Assembly and the competent organs of the United Nations" if the two countries continue to practice apartheid - legally enforced racial separation.