Joshua Nkomo, leader of one of the two guerrilla factions fighting against Rhodesian Prime Minister Ian Smith's [WORD ILLEGIBLE] government, asserted yesterday that his forces are gaining the upper hand and that time is fast running out on chances for reaching a negotiated settlement.

In a whirlwind day, which included talks with Deputy Secretary of State Warren Christopher and David Aaron of National Security Council, Nkomo appeared to be trying to present an image of being reasonable and open to continued negotiations in the 13-year-old independence fight while realizing that time was on his side.

The leader of the Zimbabwe African People's Union (ZAPU), apparently buoyed by reports from Rhodesia and Smith himself that things were not going well for the transitional government, said in an interview that a conference of all parties suggested by the United States and Britain was "getting out of date."

Nkomo maintained that the interim government established by Smith and three black leaders living in Rhodesia was "crumbling, but we are still prepared to talk with these crumbling chaps." At another point, exuding confidence, Nkomo said: "Now that they are getting finished what is there to talk about. [Should we] talk to straw?"

He added that there was "no room for compromise or for a coalition" with Bishop Abel Muzorewa or the other two "internal" leaders who made the deal with Smith three months ago.

Referring to American concern about the possibility of Soviet-Cuban intervention in Rhodesia, Nkomo said: "There is no danger of Cubans or Soviets crossing borders to fight our war. None. Zero." He denied having any Cuban advisers in Zambia, where his forces are headquartered, but there have been frequent reports that Cubans are training his ZAPU guerrillas in Angola.

He admitted that his forces receive weapons from the Soviet Union and Cuban said. "We would use weapons from this country [the United States] if it were generous enough to offer them."

He told a luncheon meeting which included State Department officials that "concern about the Communists is constantly worrying the Western world. You should just leave us alone. We are not going to do what [outsiders] want us to do," referring to both the East and the West.

Nkomo was less forthcoming about his relations with Robert Mugabe, the leader of the Muzambique-based Zimbabwe African National Union. ZANU forms the other half of the Patriotic Front which is carrying on the guerilla war for independent, black-ruled Zimbabwe, the African name for Rhodesia.

Nkomo would not say what form of government he envisaged for Zimbabwester.He added that the two groups would nor who he thought might be prime mini-"go to electtions as one and were not going to fight over who would be prime minister." Mugabe, a Marxist, is generally regarded in Western eyes as the most radical of the potential leaders of the country.