The five frontline African states began a hastily convened summit meeting yesterday with the external black nationalist leaders of Rhodesia and Namibia to discuss the uncertain fate of Western peace proposals for the two southern Africa flashpoints.

U.S. Ambassador Andrew Young has come here to lobbly for the Western plans.

One objective of the summit appears to be a reaffirmation of the frontline countries' support for the Anglo-American proposals on Rhodesia following the signing earlier this month of the agreement between Prime Minister Ian Smith and the three internal black nationalist leaders.

The five African presidents are reportedly anxious to keep the United States and particularly Britain, which has shown signs of wavering, committed to their own proposals and to prevent them from swinging around in favor of the Salisbury accord.

Attending the meeting are four of the five frontline presidents - Julius Nyerere of Tanzania, Kenneth Kaunda of Zambia, Seretse Khama of Botswana and Samora Machel of Mozambique - as well as the two coleaders of the externally based guerrilla alliance, the Pastriotic Front, Robert Mugabe and Joshua Nkomo.

It was not immediately know why the fifth president, Agostinho Neto of Angola, was not present, but his country was being represented by a diplomat.

Also participating is Sam Nujoma, leader of the Southwest Africa People's Organization (SWAPO), th emilitant nationalist group in South African-administered territory of Namibia.

U.S. diplomatic sources said Washington had hoped to have ready the final proposals for a compromise to the Namibia dispute so that the frontline presidents could exercise a moderating influence on SWAPO here. The five Western powers drawing up the proposals have not yet reached final agreement, however.

Nonetheless, Young told reporters in Lusaka, Zambia, earlier last week that he is more hopeful of progress right now on the Namibia negotiations than on those involving Rhodesia. In the latter case, one of the parties has shown interest in the U.S. idea of a conference bringing together the internal and external nationalist leaders.

About the most the Americans can expect from the frontline presidents is that they will urge Nujoma to make sufficient concessions in the dead-locked negotiations over Nambia to prevent South Africa from pursuing its own internal settlement. The South Africans are known to be seriously considering the holding of their own elections in the territory and turning their back on the Western plan for including SWAPO and bringing in the United Nations to oversee the vote.

U.S. diplomats seem to be hoping that if South Africa can be persuaded to support the Western proposals for the Nambia conflict then it will also eventually back the Anglo-American plan for Rhodesia and apply pressure on Smith to accept it.

The summit here is expected to last only two days. On Monday young is scheduled to fly to Arusha in northern Tanzania and then go by car to Nairobi, Kenya, Tuesday. From there he is to fly to Lagos to meet up with President Carter for the President's visit to Nigeria and Liberia.