Black Rhodesian nationalist leaders indicated clearly here today that they are opposed to one of the key provisions of the latest Anglo-American plan for settlement of the Rhodesian dispute, raising fresh doubts about the future of the British-American peace initiative.

Joshu Nkomo and Robert Mugabe, co-leaders of the Patriotic Front, the main nationalist organization, in effect rejected the establishment of a neutral international peacekeeping force in an interim period before elections for a majority government in Rhodesia.

After 2 1/2 hours of talks with British Foreign Secretary David Owen and Andrew Young, U.S. ambassador to the United Nations, the two nationalist leaders said their position remained unchanged. "Our situation is a war situation," said Nkomo."Any discussions must take that as a fact so that any movement from the war to independence must be superintended by those forces that are fighting for change."

The Patriotic Front is a coalition of nearly all the guerrillas fighting in Rhodesia and based in camps in neighboring countries. It has the backing of the five African "front-line states" and of the Organization of African Unity. In addition, the Front's army yesterday received the endorsement of the "front-line states" as the sole basis for a new security force in Rhodesia after the advent of majority rule. At present, the breakaway British colony is governed by representatives of the 270,000 whites while most of the 5 million, blacks have no voice in government.

The Anglo-American proposals are understood to call for a neutral international force in the transition period leading to majority rule and the dismantling of parts of both the white-dominated Rhodesian army and the guerrilla force before a new mixed security force is formed.

Despite Nkomo's insistence on a dominant position for the Front's army in the transition period, neither he nor Mugabe rejected the British-American plan outright today. They said they still did not know the exact details of the proposals and indicated that they would not make known the Front's official reaction until their publication at the end of this week.

Owen, anxious to prevent the two nationalist leaders from commenting further, cut into their exchange with reporters to say, "We are not going to say anything more."

British and American-members of the joint delegation have been anxious to avoid any public polemics at this critical juncture in the latest search for an internationally acceptable solution to the 17-year-old guerrilla war and constitutional dispute in the former British colony.

Later, upon arriving in Johannesburg, South Africa, Owen was again reluctant to say much about the peace plan or what he hoped to achieve in his talks Monday with South African Prime Minister John Vorster.

Asked what role he expected South Africa to play in the increasingly difficult search for a negotiated settlement, Owen said he believed it was "very strongly" in South Africa's interest to help find a solution.

Observers here are skeptical that Vorster will agree to pressure the white minority Rhodesian government [TEXT OMITTED FROM SOURCE]