Angola and South Africa have put a de facto cease-fire into effect along their war-torn frontier, and a formal disengagement pact is expected soon as a result of direct negotiations between the two countries, according to U.S. officials.

The effect of the pact would be to push Cuban troops stationed in Angola and African guerrillas raiding into the disputed territory of Namibia out of the war zone and to halt the punishing raids into Angola by South African troops. The South African raids, rather than a highly publicized U.S. diplomatic drive in the region, appear to have brought the Angolans to the negotiating table, diplomats say.

Because the South African-Angolan negotiations apparently do not provide for any further steps toward independence for Namibia or eventual withdrawal of Cuban forces from Angola, Western and African countries involved in the protracted multinational negotiations for a Namibian settlement are concerned that the bilateral cease-fire could stalemate further progress on Namibia.

The fear, which also has been expressed privately by some U. S. officials, is that agreement with Angola will reduce pressure on South Africa to grant early independence to Namibia, also known as South-West Africa.

The South Africa-Angola accord first was discussed at a meeting between the two governments in the Cape Verde Islands last December.

The main points of the disengagement plan include a freeze on all military activity, a withdrawal of the Cubans to a line 185 miles north of the Namibian border and a pullback by the Namibian guerrilla movement South-West Africa People's Organization (SWAPO) even farther than that, in return for the withdrawal of South African forces from positions they occupy in Angola back across the Namibian border.

The key role of Jonas Savimbi's dissident Angolan guerrilla movement UNITA is not mentioned, but the Angolans are believed to be seeking assurances that South Africa would end its military support for the movement.

The disengagement plan is less a triumph for American diplomacy than for South Africa's tactics of bombing its opponents to the conference table, foreign diplomats and congressional sources said. The combined pressure of South African forces scouring southern Angola for SWAPO guerrillas and the disruptive activities of UNITA, they said, convinced the Angolan government that it had to seek a breathing space.

A formal cease-fire and disengagement are likely to be publicly praised by the Reagan administration as a necessary preliminary move in its game plan for a Namibian solution. But diplomats involved in the negotiations said it will not remove doubts that Angola and particularly South Africa, will have less incentive than before in pursuing Washington's grand design in southern Africa before the administration enters the 1984 election campaign. The most critical part of the policy is the linked removal of the Cubans from Angola with South African withdrawal from Namibia, accompanied by U.N.-supervised elections in the territory.

The United States has been privy to, but not part of, the Angolan-South African dialogue. The administration has kept its own lines open to Luanda on a second track but appears, sources said, to have made considerably less progress on the broader issues than the two direct antagonists have achieved on their more pressing local concerns.

Pretoria's tactics in Angola are seen as part of its general destabilization strategy in southern Africa. It has already chalked up successes with Zimbabwe, Mozambique and Lesotho, in terms of limiting their political maneuverability and capacity to aid guerrillas opposing the South African government, by using a judicious if ruthless mixture of cross-border raids, support for insurgents fighting the black African governments and readiness to talk.

A crucial factor in pressuring the Angolan government to reach an agreement has been the recent military successes of UNITA. Backed heavily by South Africa and operating from a secure tribal base in southeastern Angola, Savimbi's guerrillas launched a sustained Christmas offensive, striking farther north than ever before.

Each side will obtain something of substance out of the disengagement agreement, informed sources said. Angola will be relieved of relentless South African pressure. South Africa will be assured that SWAPO is too far from the Namibian border--and under Angolan control--to be a serious nuisance.

The withdrawal of the approximately 20,000 Cuban troops stationed in Angola since the 1975-76 civil war has been described by American diplomats privately as an overriding objective of the Reagan administration's regional policy and its emphasis on negotiating a Namibian settlement.

The Cubans, who have not represented a direct military threat to South Africa, were always hurriedly pulled back whenever the South Africans advanced. Under the agreement reportedly under negotiation, the Cubans will be tucked away even more securely.

South African officials have repeatedly emphasized that complete evacuation of the Cubans as a key to a Namibian settlement was an American initiative, and was a problem to be solved by the Reagan administration.

Diplomats from the four Western countries--Canada, Britain, France and West Germany--that joined the United States in trying to end South African occupation of Namibia recognize that the interim pact will lead to a lessening of tension.

There is, however, a joker in the pack, since Savimbi's role remains unclear. His forces will stay in the cease-fire zone, a threat to the Angolan government and an option available to South Africa should it feel the military freeze is not as solid as it should be. The Popular Liberation Movement of Angola government, which has been at war with UNITA since independence in 1975, shows no signs that it is ready for a reconciliation. The need for the Cubans is thus likely to be as great as ever.

South Africa similarly shows few signs of being in a hurry to pull out of Namibia where the pro-Pretoria internal coalition recently collapsed. Prime Minister P. W. Botha fears right-wing accusations of selling out in Namibia if he fails to produce a credible political force to oppose SWAPO in U.N.-supervised elections.

The problem for the administration, as described by U.S. and foreign officials, is that South Africa's agenda is sometimes different from Washington's. Pretoria's policy is multilayered and complex, with the Foreign Ministry, the military and military intelligence often acting independently. The government has its own way of dealing with regional problems, sometimes tempered by the restraining hand of the United States, as in Zimbabwe.

State Department officials stress the comprehensive scope of U.S. policy in southern Africa, citing improved relations with Mozambique, a decision to supply Botswana with a limited amount of arms and the new dialogue with Angola. They argue that the United States has moved into a position of honest broker, with the support of the black governments in the region--regardless of what they say in public--thereby diminishing Soviet influence.

Congressional and foreign critics of the administration's "constructive engagement" policy say that South Africa often misreads the signals from Washington, that violence, especially South African-initiated, has increased markedly since Reagan came to office, and that the Cuban "linkage" tactic has derailed the Namibian negotiations.