Rhodesian Prime Minister Ian Smith and guerrilla leader Joshua Nkomo acknowledged yesterday that they had met secretly in Lusaka, Zambia, three weeks ago to discuss a role for the Patriotic Front guerrillas in a Rhodesian settlement.

Nkomo told newsmen in Lusaka that Smith said "as far as he was concerned, he was out," Nkomo said the white Rhodesian leader told him he "definitely would be prepared to see power transferred . . . to the Patriotic Front."

Smith, in a statement issued in the Rhodesian capital of Salisbury, emphatically denied that he had offered to hand over power, but, acknowledged he had held "exploratory" discussions with the guerrilla leader.

Nkomo said the question of further meetings with Smith was being discussed by the Patriotic Front leadership. His coleader in the Patriotic Front, Robert Mugabe, did not attend the Lusaka meeting. Observers have long felt that Smith might ultimately try to split Nkomo away from Mugabe by offering Nkomo the leading role in a black government.

Washington Post correspondent David B. Ottaway reported that confirmation of the rumors, which had been circulating in Salisbury, of a Smith-Nkomo meeting was bound to have an enormous impact on Rhodesia's multiracial transitional government, and could even lead to its collapse.

By meeting with Nkomo, Smith in effect conceded that the three black leaders with whom he negotiated his "internal settlement" earlier this year - Bishop Abel Muzorewa, the Rev. Ndhaningi Sithole and Chief Jeremiah Chirau - are incapable of halting the escalating guerrilla way, Ottaway reported.

Muzorewa and Sithole had both assured Smith at the time of the "internal settlement" that they had strong support among the estimated 20,000 Patriotic Front Guerrillas, and could bring the fighting to an end.

Although Smith said yesterday that his three black partners in the interim government were informed in advance of his meeting with Nkomo, Muzorewa and Sithole reacted angrily to reports that he had offered to hand over power to their chief rival.

Sithole insisted that he had not been informed of the Lusaka talks in advance, and said any effort by Smith to transfer power to the Patriotic Front would cause great instability and confusion in Rhodesia.

"As an individual, Mr. Smith hasn't the power and therefore he cannot hand it over," Sithole said.

A bitter Murozewa declined any direct comment on the meeting. "After Mr. Smith has declared to the public and the world that he is completely defeated militarily and that he is surrendering, I will react," he said.

Smith's statement yesterday expressed surprise that Nkomo - who initially denied reports of the meeting - later confirmed it had taken place. Smith said the talks had been kept secret at the request of Nkomo and Zambian President Kenneth Kaunda, the host, "because neither Mr. Mugabe nor the frontline presidents were aware that it was to have taken place."

Smith said Kaunda had invited him to the meeting, which was also attended by former Nigerian foreign minister Joseph Garba.

If any progress had come from the talks, he said, he and his three black colleagues in the transitional government would have acted jointly on it.

Nkomo said Smith had initiated the meeting, and had offered to transfer power in order to end the war and preserve stability.

"He wanted to see a stable government in Zimbabwe (the nationalist name for Rhodesia) and could the Patriotic Front do something," Nkomo said.

United Press International reported that Nkomo later said at his home in Lusaka that if Smith meant business, the Patriotic Front would be prepared to make a deal with him.

"But that means a visible transfer of power," he said. "It means the complete wiping out of the present setup."

Nkomo's comments followed more than nine hours of talks in Lusaka yesterday with the presidents of the frontline states, Zambia, Tanzania, Botswana, Angola and Mozambique.

The talks reportedly centered on ways to reconcile differences between Nkomo and Mugabe, who claims his supporters have born the brunt of the guerrilla war, and therefore deserve major concessions in any settlement.

Nkomo is known to be keeping the bulk of his forces in Zambia while the bulk of Mugabe's are fighting in Rhodesia.

The confirmation that the talks with Smith took place and Mugabe's absence from them are certain to arouse additional suspicion within Mugabe's wing of the Patriotic Front and increase the growing antagonism betwen its two factions.

Mugabe is known to suspect Nkomo is holding back his forces in preparation for a later power struggle with Mugabe's.

The meeting of the frontline states ended without the statements the presidents normally issue.

News service reports from Lusaka indicated the meeting was stormy, and involved pressure on Mugabe and Nkomo to soften their conditions for a settlement in view of increasing apprehension among the frontline states over the possibility of protracted war such as the civil war that tore apart Angola in 1975.

Kaunda also reportedly came under pressure for acting without the consent of the other frontline leaders in meeting with Smith.

Kaunda was the host of another secret meeting between Smith and Nkomo last year several months before Smith began the talks with the Muzorewa, Sithole and Chirau that led to the transitional government and the internal settlement.

Under the terms of that settlement, elections including all black adults are to be held by the end of this year.

But increased guerrilla activity in the countryside, and lately within Salisbury itself, cast doubts on the possibility of holding the elections.

Smith has recently been critical of Sithole and Muzorewa for their failure to control the guerrilla activity, charging them with failure to hold up their end of the bargain in the settlement.

He is considered receptive to Nkomo, with whom he has previously tried to strike settlement deals.

The strains between Nkomo and Mugabe appear sporadically. Last month, Mugabe abruptly left talks the two were conducting and flew to Nigeria with his key military colleague, Josia Tongagara. The little-noted trip occurred around the time of the meeting between Nkomo and Smith.

Nigeria has played a key role in backing the frontline states, and a Nigerian representative reportedly attended the meeting in Lusaka.

Mugabe recently accused the Americans and the British of cooperating with Smith in "secret maneuvers" aimed at imposing their choice of a head of state on Rhodesia. The accusation was interpreted as an oblique criticism of Nkomo.

The British and Americans have been trying to bring all sides in the Rhodesian dispute together at a conference.

Smith, Chirau, Nkomo and Mugabe have agreed to participate, although each has offered qualifications. Sithole and Muzorewa have adamantly refused.