African "front-line" leaders warned the United States and Britain yesterday that the time had come to fully support their own Anglo-American proposals for Rhodesia's future or publicly renounce them.

The African leaders said after a summit meeting here with guerrilla leaders opposed to the new multi-racial government in Salisbury that conditions now call for an intensification of the "just armed struggle for the liberation of Zimbabwe," the nationalist name for the breakaway British colony.

"If they still support these proposals, they should move ahead and convene in the shortest time possible a meeting to follow up what was agreed to Malta," the African leaders said in a joint communique referring to the talks held between the two Western powers and Patriotic Front guerrilla leaders on the island of Malta in January.

"If on the other hand they have decided to abandon their commitment to their own proposals for which they had requested and obtained the support of the Patriotic Front, the frontline states tand the international community, they should so declare without any further delay."

Altogether, the summit did not seem to have advanced the search for a breakthrough in the deadlocked negotiations over either Namibia or Rhodesia. In the latter case, Western diplomats here seemed to feel that it is too early to expect any dramatic development until the real strength or weakness of the new transitional government in Salisbury becomes clearer.

Nevertheless, the statement issued yesterday appears to leave the next step in the tension-ridden conflict up to Washington and London.

The four-page communique left no doubt that the African leaders rejected the multiracial government formed last week in Salisbury by Prime Minister Ian Smith and three internally based black nationalist leaders, something the United States and the British have not done. The Anglo-American initiative has sought to bring the externally based guerrilla leaders into the negotiating process for Rhodesia's future.

While the frontline and guerrilla leaders did not reject an American proposal for a new peace conference bringing together Rhodesia's internal and external nationalist leaders, neither did they indicate any support for it.

Instead, the frontline stares reaffirmed their "total and unwavering support" for the Patriotic Front and called on all "progressive and anti-colonialist states" to support the struggle against the multiracial interim government established last Tuesday, including a call for wide U.N. sanctions.

There was no mention of Namibia in the communique. It was primarily the Namibia issue that brought U.S. Ambassador to the United Nations, Andrew Young, here for the summit meeting.

Young's first reaction to the results of the meeting here was that the front-lines states had issued "a challenge" to Britain and the United States but that the the communique contained no specific suggestions as to how the Western powers should go about convoking another meeting on the British-American proposals in the present circumstances.

"It is in fact the details that are most important in terms of where we go from here," he said, hinting that a more detailed explanation of what the five front-line states had really decided would be conveyed shortly to the two Western governments through private channels. "We have to negotiate privately," Young commented.

Earlier, U.S. diplomats said they believed the front-line presidents were moving toward an endorsement of the American proposal for an all-parties conference but only if it were based on the British-American proposals. It was later learned that they were urging first a reconvening of the Malta conference to reach agreement with the British and Americans on outstanding military issues before any larger meeting involving all the parties to the Rhodesian dispute is held.

Observers here felt it was questionable that Smith and the three internally based nationalist leaders who signed the Salisbury agreement would be interested in such a conference under these terms, leaving a diplomatic impasse in the whole British-American initiative.

Reflecting on the confused state of both Rhodesia and Namibia, Young said, "The pieces will either fall into place before too long or the whole thing will fall apart for some time to come."

Young met early yesterday with Tanzanian President Julius Nyerere to be briefed on the progress of the conference. But he failed to see the three other presidents attending: Samora Machel of Mozambique, Seretse Khama of Botswana and Kenneth Kaunda of Zambia. The fifith president, Agostinho Neto of Angola, did not attend - apparently because of pressing problems at home.

Young had talked earlier in Lusaka with President Kaunda. Joshua Nkomo of the Patriotic Front and Sam Nujoma, president of the Southwest African People's Organization, the militant nationalist group in Namibia.

The U.S. ambassador had hoped that the front-line states could play a role in pressing Nujoma to agree to the final Western proposals for a settlement in Namibia but they are still not ready.