Exiled leaders of the main underground movement fighting white-minority rule in South Africa have indicated that while the group is maintaining its hard-line public stance against negotiating with the government, behind the scenes it is trying to form a broad-based alliance that can pressure Pretoria into negotiating with blacks about a new constitutional future.

Thabo Mbeki, publicity director of the African National Congress (ANC), disclosed in an interview here that the organization was making contact inside South Africa with a wide range of groups, ranging from worried businessmen and white Afrikaner intellectuals to disillusioned blacks holding office in the tribal "homelands" under the apartheid system of segregation.

"We are reaching out, exploring all these forces, trying to mobilize the political weight of all of them to the point where Botha finds that he can't go on confronting them in their totality and he says, 'Okay, let's talk,' " Mbeki said, referring to South African President Pieter W. Botha.

"At that point the ANC would be prepared to negotiate," added Mbeki, one of the rising younger generation within the congress who is widely regarded as a candidate for top leadership ranks.

What Mbeki described is a completely new strategic approach for the congress, which for the past 25 years has been committed to the one-track course of trying to overthrow the government by revolutionary armed struggle.

Only last June at a congress in Kabwe, Zambia, the congress reaffirmed that it was not prepared to seek a political settlement through negotiation, and would only negotiate a handover of power by a defeated white regime.

A constitutional convention, or equivalent of the 1979 Lancaster House conference at which a new constitution was negotiated transferring power from Rhodesia's whites to the black majority of what is now Zimbabwe, was also ruled out.

Formally, that position is little changed. In a lengthy interview here, ANC President Oliver Tambo repeated conditions that must be met before his organization would even consider "talks about talks." These include the unconditional release inside South Africa of imprisoned leader Nelson Mandela and others serving life sentences with him, the return of political exiles and a lifting of the ban on the congress and all other restrictions on political activity.

At the moment there was "absolutely nothing" to suggest that Pretoria was ready or willing to negotiate, Tambo said.

At the same time he was adamant that he would not meet Botha's single stipulated condition for talks, that the organization first renounce violence.

"We think that is an unreasonable condition," Tambo said. "Apartheid constitutes an act of violence against our people, and for us to declare a moratorium . . . while . . . apartheid continues would mean that we were acting unilaterally."

But while the formal positions remain deadlocked, the congress is actively working behind the scenes to try to increase domestic political pressures on the Botha government to force it to the negotiating table. Mbeki explained that the congress is prepared to be highly pragmatic.

"The call you make to the people is the same, saying you are not prepared to negotiate, that you must intensify the offensive. But in the course of that offensive it is clear that one of the important things that is happening is the beginning of a breakup of the power structure," Mbeki said.

To underline his point, Mbeki noted that he had just returned from talks in London with the white former opposition leader in the South African Parliament, Frederik van Zyl Slabbert, who resigned last month to try to play a mediating role between the congress and groups in South Africa concerned about the country's future.

Mbeki, Tambo and other members of the congress national executive were also about to begin a series of talks with the leader of the Kangwane tribal "homeland," Enos Mabuza, who arrived here with 20 members of his "cabinet."

The underground organization also has held meetings recently with white Afrikaner students and churchmen, as well as with several groups of businessmen, including the head of the Anglo-American mining corporation, Gavin Relly.

"Out of all this I think you will get a realignment of forces," Mbeki said. "If we can mobilize them and form some kind of alliance with them, then we are talking not of overthrowing the government but of turning so many people against it that it would be forced to do what Ian Smith had to do" in Rhodesia.

Meanwhile, the congress would step up the pressure by intensifying its guerrilla struggle and organizing mass action through strikes and consumer boycotts, Mbeki said.

The widespread township unrest, now in its 19th month, has served to create "combat units" in these ghettos that were a "potential extension of our guerrilla army," Mbeki said.

"We are moving close to our objective of waging a people's war of liberation," Mbeki said.

It is clearly a two-track strategy -- intensifying the physical pressure on the white power structure to increase its inner doubts and divisions, while trying to take advantage of these divisions to build an alliance broad enough to pressure the administration into opening the door to negotiations.

In pursuing this strategy, the congress does not claim to be in direct control of the ongoing rebellion in the black townships. But it does claim that the mass demonstrations are broadly supportive of it, a view endorsed by reporters who have seen the enthusiastic displays of ANC banners and heard the massed crowds chanting slogans in praise of Mandela, Tambo and the guerrillas.

The pragmatism of the new approach is surprising to some observers who have regarded the congress as tending to be rigid and doctrinaire.

The decision to talk to Mabuza was in itself remarkable. The "homeland" leaders are regarded as the ultimate collaborators with the apartheid administration, and activists inside South Africa label anyone holding office in "the system" as a "sellout," to be shunned and sometimes even killed.

The ANC has characterized Mabuza as "an exception" because of his criticisms of the Botha government, but spokesmen in Lusaka made it clear that he was not necessarily a solitary exception.

"It is the view of the ANC that whatever forces on the ground in South Africa, in or out of the system, want to have a dialogue with us, we are willing to see them," Jordan said.

But the organization draws a line, for the moment at least, at Chief Gatsha Buthelezi, the moderate Zulu leader. Buthelezi and ANC leaders, seeming to see each other as rivals, have engaged in a bitter war of words in recent years and Mbeki said, "I think he has made himself inaccessible to us."

Nor does the wooing of dissidents go so far as to offer constitutional safeguards for the white minority. "That would be a mistake," said Tambo. "If we perpetuate the identification of races as separate groups, then we will really be perpetuating the status quo."

Instead, said Tambo, the congress laid emphasis on the individuality of people regardless of race, color or creed.

"Our freedom charter recognizes that the white people are part of the South African nation, despite their different origins," he said. "We see them simply as people, fellow citizens who belong there with the rest of us, and we will all join together in running and building our country."

"We have to get that idea across," Tambo added. "If whites see themselves as a minority in a democratic South Africa it is only because they see themselves as separate. That is a product of apartheid. We have got to fight this concept of racism. We must destroy it."

Tambo said he believes the ANC would be able to get this point across to white South Africans if it had access to them, but the banning of the organization and the prohibition on quoting its leaders inside the country made this difficult.

Referring to South African government allegations that the ANC, which has several members of the country's proscribed Communist Party on its national executive, is controlled from Moscow, Tambo insisted this was not the case.

"Yes, we do have some Communists on our executive, I have never counted how many, but they all owe their primary allegiance to the ANC and that's all that matters to me," Tambo said.