Two top Reagan administration officials said yesterday that progress is being made in international efforts to obtain the release from prison of black South African nationalist leader Nelson Mandela and the start of negotiations between white and black leaders of that country.

Secretary of State George P. Shultz, speaking to a group of American religious leaders, defended the U.S. policy toward South Africa of "constructive engagement," which calls for quietly pressuring Pretoria for reform. Shultz, citing reports from mediators, also said Mandela and other South African black leaders are willing to enter talks with the white apartheid regime "if there was a reasonable chance of something genuinely substantive coming out of it."

The mediators are seven members of the so-called Eminent Persons Group, former leaders of the Commonwealth group of nations, who have been talking to the South African government and exiled or imprisoned South African black leaders, including holding lengthy discussions with Mandela.

Deputy Assistant Secretary of State Chester A. Crocker told the same group that the Pretoria government was "in fact eager" to find a formula for Mandela's release and was no longer insisting that he forswear violence as a precondition.

Shultz and Crocker spoke as South Africa again denounced the African National Congress, charging that it "does not comply" with the government's demand that violence be abandoned. In a booklet titled "Talking With the ANC," the government said the organization had an alliance with the South African Communist Party, and "both are committed to the violent overthrow of the present system of government."

The Eminent Persons Group is urging Pretoria to legalize the African National Congress, which Mandela leads, and to free him and other imprisoned black leaders. In return, the ANC would declare a truce in its guerrilla struggle and work to end the current violence in South Africa's black townships.

But Shultz suggested that the Eminent Persons Group "maybe will decide" to drop its mediation efforts after South Africa's raids two weeks ago against ANC sites in three neighboring black countries.

In defending U.S. policy, Shultz said the U.S. attitude is that "we need to be engaged somehow or other . . . . And certainly in our efforts we want to be constructive. So I don't frankly see what's wrong with that."