The United States has received a "substantive reply" from the South African government to its proposal to settle the conflict in southern Africa that would allow resumption of U.S. mediation efforts, a senior administration official said yesterday.

The reply, details of which were not disclosed, was discussed with Angolan government representatives by Assistant Secretary of State Chester A. Crocker during talks held Nov. 27-28 in Lusaka, Zambia.

Urging South Africa and Angola to reject a further escalation of fighting, the official said, "This clearly is a moment in which there is a chance for the parties to grasp an opportunity before the window closes down."

Last March, the United States offered a compromise on the timetable for withdrawal of 35,000 Cuban troops from Angola tied to the start of U.N.-supervised independence elections in South African-ruled Namibia.

But Angola suspended the talks after South African raids into Angola and Botswana and Congress' decision to end a ban on U.S. aid to noncommunist rebels fighting the Marxist Angolan government.

The official said the Lusaka talks with the Angolans, the first in five months, had been "positive in tone." He said further talks will be held soon and urged Angola "to act decisively" so a breakthrough in the five-year negotiations can be made.

The official noted "some impatience" in Washington with lack of progress in the talks and "considerable sympathy" for providing U.S. aid to the rebels. But he refused to comment on administration plans to begin a covert-aid program soon if no progress is made in the new round of talks.