U.S. Ambassador Andrew Young said at the United Nations yesterday that he personally favors some form of sanctions against the government of South Africa, which carried out a sweeping purge of black leaders and anti apartheid organizations last week.

Young who conferred with President Carter earlier in the day in Washington stressed that this was a personal opinion and added: "The President and Secretary of State will have to decide what sanctions are appropriate in these conditions."

The U.N. envoy said he expects the Carter administration to make its decision within the next two days.

Young made his comments immediately following a one-hour. Security Councl debate during which four resolutions including arms embargo on South Africa, were proposed by black Africa nations. No vote is expected on any resolutions until later in the week.

U.S. sources at the United Nations indicated that the Carter administration, which had promised last spring to review its relations with South Africa if Pretoria did nothing to alter its internal racial policies was considering more moderate measures than economic sanctions against South Africa.

"There is ample evidence that our policy of week reasonableness has not worked," the source said. The measures the Carter administration might back would include discouraging new investments in South Africa and ending government credit guarantees for investments and loans to that country, he said.

Young's expression of support for sanctions against South Africa, at a time when pressure is building for some sort of international rebuke to Preforia for its crackdown on dissidents, could complicate the Carter Administration's efforts to soften black African demands for extreme measures.

It also represents a hardening of the U.N. ambassador's own position on the question. In the past, Young has spoken of international economic sanctions against South Africa as a "non violent weapon analogous in some ways to the sit-ins and boycotts" preferred by civil rights workers in the U.S. South.

Yesterday marked the first time in his capacity as U.N. ambassador that he has come out explicity for some kind of mandatory sanctions against the racially troubled African country. He would not elaborate on what kind of sanctions he favors.

When mandatory arms or economic sanctions against South Africa have been proposed at the United Nations in the past, the United States along with its major European allies. Britian and France, has vetoed such action.

The United States and Britain currently abide by a voluntary arms embargo against South Africa.

Sources said Young has spent much of the weekend attempting to find an approach to the South African question that would be acceptable to other Western and black African nations, and one that the Carter administration could support.

On Friday, he met twice with representatives of the four other Western nations on the Security Council - Britain, France, West Germany and Canada - and then flew to Washington for consultations Saturday at the State Department.

The Security Council debate, scheduled to run four days was requested by 49 African countries after South Africa banned virtually all important black organizations, closed two black-operated newspapers and arrested some 50 black leaders last Wednesday.

Speaking on behalf of the African group, Ambassador Mahmoud Mestiri of Tunisia said the time had come for the council to take steps beyond issuing considerations of the South Africans.

"Events of the past few days," he said, referring to Pretoria's crackdown on dissent, "have exposed the true intentions of South Africa."

The only other speaker in the meeting was David Sibeko, representative of the Pan Africanist Congress of Azania, a black liberation movement that is banned in South Africa.

If the major Western powers and the African group cannot negotiate a general condemnation of South Africa and instead must vote on an arms embargo or economic sanctions, the Carter administration will be faced with a difficult choice.

Vetoing such proposals could unravel much of the goodwill Young has been able to build up in black African countries, but voting for such severe measures would antagonize South Africa, whose support the Carter administration has sought for a peaceful transition to majority rule in Rhodesia and Namibia.

One Western diplomatic source viewed the call for a mandatory arms embargo as an "opening bid."

"We haven't yet begun to negotiate with the Africans on this," he said.