Assistant Secretary of State Chester A. Crocker held talks with South African leaders today and later expressed continuing optimism in the Reagan administration's drawn-out attempts to negotiate independence for Namibia.

There was no sign of any breakthrough, however, on the issue which has stalled the negotiations -- South Africa's demand, supported by the United States, that the estimated 20,000 Cuban troops in neighboring Angola must withdraw before there can be an agreement, and Angola's insistence, supported by other African states in the region, that the two issues cannot be linked.

South African Foreign Minister Roelof F. Botha expressed his satisfaction at a statement by Vice President George Bush in Harare, Zimbabwe, last night reaffirming U.S. support for the South African position.

Bush's statement was delivered at an official banquet hosted by Zimbabwe Prime Minister Robert Mugabe. The Zimbabwean leader had just made a strong appeal for the Reagan administration to support the African viewpoint.

Botha described Bush's statement as "very strong and made under difficult circumstances."

Crocker broke away from Bush's seven-nation African tour to fly here for talks with the South African leaders in an effort to breathe new life into the negotiations.

Crocker's optimistic statement came when reporters, after referring to Bush's Harare statement, asked whether the assistant secretary did not think the negotiations were deadlocked.

"I do not," Crocker replied promptly. "The negotiations are continuing and none of the parties have indicated that they wished the effort to come to a halt."

Present with Botha at today's meeting was Defense Minister Magnus Malan, a rising force in the South African Cabinet and a hard-liner on the strategic value to South Africa of retaining control of Namibia.

In a speech to a businessmen's breakfast in Cape Town last month Malan was reported as saying he was certain the South-West African People's Organization (SWAPO), the main guerrilla group fighting for Namibian independence, would win any elections held now, and that "I believe SWAPO is communist."

If South Africa withdrew from Namibia, the red flag would fly over Windhoek, the Namibian capital, and the "operational area" -- South Africa's term for the war front with African nationalist guerrilla forces -- would shift from southern Angola to southern Namibia, Malan said.

This would result in Botswana and Zimbabwe falling to the communists, he declared. Malan said he regarded Mozambique as already communist, and thus all of South Africa's northern borders would be opened up to the "terrorists."

After South African newspapers interpreted this speech as meaning the government was no longer prepared to accept a SWAPO election victory in Namibia, Malan insisted he had not articulated any new policy.

Despite this, the impression remained that Malan's strategic assessment cast a new light on South Africa's intentions, and strengthened the view here that there is no longer any chance of an early settlement.