PORTLAND, ORE., JUNE 20 -- The U.S. ambassador to South Africa, appearing to break with Reagan administration policy, applauded U.S. sanctions against the racially segregated nation and said he would not discourage companies from leaving it.

In his first public speech in the United States since becoming ambassador, Edward Perkins said the sanctions have been "an unmitigated success" in expressing contempt for apartheid. However, he said it is too early to tell whether they will change apartheid.

President Reagan vetoed sanctions legislation last year, but the veto was overridden.

Perkins said there is a need for "more inventive ways to manage our relationship with the government of South Africa and with the South African people."

"We have to make it abundantly clear in almost any way we can our abhorrence of a system which has a minority of the population enjoying economic and political rights at the expense of the majority," he told a joint meeting of the City Club and the World Affairs Council of Oregon on Friday. Perkins is from Portland.

Although the administration opposes the departure of American companies from South Africa, Perkins said, "I don't urge companies to leave or stay." He said it is "far beyond my capability to address" the issue of divestment, referring to the debate over whether local and state governments, colleges and other groups should rid themselves of stock in companies doing business with South Africa.

Perkins said companies that stay in South Africa must remain socially engaged and those that withdraw should leave something for black South Africans. As an example, he said, universities that divest could put their money in a trust to help black South Africans.

The administration has maintained that divestment and the departure of American companies from South Africa thwart efforts for peaceful change and hurt blacks more than whites.

Perkins did not address whether sanctions are wise from an economic viewpoint, but said:

"The comprehensive Anti-Apartheid Act of 1986, as a statement of abhorrence by the American people of a hated system, was an unmitigated success. There is no question about where the American people stand with respect to South Africa and its government at this time.

"Obviously, sanctions have not brought down the government," he said. "We don't know whether the government is affected by them or not. In short, it is too soon to tell."