The Angolan government has sent a letter to the United Nations calling upon Secretary General Javier Perez de Cuellar to resume his negotiations in southern Africa and effectively ending the primary mediation role the United States has played there during the past five years.

The letter from Angolan President Jose Eduardo dos Santos, dated March 18, said Angola was "deeply outraged" by the Reagan administration's escalating military support for Angolan rebel leader Jonas Savimbi, which he said had "jeopardized its credibility as a mediator."

Dos Santos also charged that the United States had not fulfilled the terms of a secret and previously unpublicized document, which he called "the Mindelo Act." He said the document was signed by "leaders" of the Angolan and U.S. governments in January 1984, and provided for "the cessation of acts of aggression against Angola and support" for Savimbi's National Union for the Total Independence of Angola (UNITA).

Dos Santos reminded Perez de Cuellar in his letter that the United Nations had held the initial responsibility for the implementation of a 1978 resolution providing for elections in South African-administered Namibia. The Marxist Angolan leader called upon the United Nations to take "all necessary measures" now to implement it.

The Angolan ambassador to the United Nations, Paulo Figueiredo, confirmed that Angola was asking the United Nations to take over "the entire process" of negotiations over Namibia and said that "no way will the Angolan government accept the United States as negotiator in the process so long as it keeps on supporting UNITA."

At the start of the Reagan administration in 1981, the United States effectively took over from the United Nations and a group of five Western nations the delicate task of mediating between South Africa and Angola in an attempt to resolve the interwined issues of Namibia's independence and the withdrawal of an estimated 25,000 to 27,000 Cuban troops from Angola.

The Angolan decision now to revive the U.N. role thus represents a major setback to U.S. diplomacy in southern Africa. It raises doubts whether Washington, or anyone else, will be able to take advantage of the surprise South African offer on March 4 to begin implementation of the U.N. resolution on independence for Namibia Aug. 1, provided Angola agrees to a timetable for the withdrawal of Cuban troops.

The administration recently dispatched Deputy Assistant Secretary of State Frank G. Wisner to southern Africa to discuss Pretoria's offer with various governments. But Wisner did not visit Angola and returned to Washington without making visible progress.

The Dos Santos letter made no mention of a Cuban troop withdrawal from Angola, apparently because U.N. resolutions have never addressed the issue as part of the Namibian problem.

The disclosure that the United States had signed a document with the Angolan government, with which it has no diplomatic ties, surprised even close observers.

A U.S. official said that what the Angolans called an "act" was actually a "memorandum of understanding," what diplomats call an "aide memoire," signed at Mindelo in the Cape Verde Islands following talks between Wisner and Angolan Deputy Foreign Minister Venancio de Moura in January 1984.

The document spelled out what the two sides understood to be an agreed course of action, according to both sides.

The Angolan view, according to Figueiredo, was that the United States "certainly agreed" not to provide any aid to UNITA as one of its undertakings. But a U.S. official said the document simply contained the Angolan viewpoint "urging" the United States to use its influence with South Africa to stop South African "acts of aggression" against Angola and aid to Savimbi.