Southern African cease-fire survives first challenge, but the toughest test is ahead

The fledgling peace agreement between South Africa and Angola has apparently survived its first serious test. But that challenge underscored the basic weakness that makes the agreement a fragile one, diplomatic observers say.

The agreement establishes a cease-fire in southern Angola and the ''disengagement'' of South African troops from that region. South Africa has agreed to withdraw its troops as long as Angola does not permit Cuban or SWAPO (South West Africa People's Organization) forces to enter the area. SWAPO is fighting South Africa for control of Namibia (South-West Africa).

But the agreement was put in jeopardy by allegations from South Africa that SWAPO guerrillas are already infiltrating Namibia in a new attack.

The Reagan administration hopes the cease-fire and disengagement agreement will lead to independence for Namibia. Once peace is established on the border, the Reagan administration hopes Angola will make a commitment to withdraw Cuban troops from that country while South Africa makes a commitment to implement the United Nations plan for Namibian independence.

One weakness of the agreement is that SWAPO remains suspicious that the agreement could lead to a rapprochement between Angola and South Africa that does not include Namibian independence.

SWAPO leader Sam Nujoma has said he would honor the cease-fire in southern Angola but that the ''struggle'' within Namibia would go on until SWAPO and South Africa signed their own cease-fire.

SWAPO's effectiveness will be greatly reduced without access to bases in southern Angola, military analysts say. But other observers expect SWAPO to try to maintain whatever military pressure it can in Namibia to ensure that South Africa begins cease-fire negotiations with SWAPO.

Another possible threat to the agreement is the UNITA (National Union for the Total Independence of Angola) rebel movement within Angola.

UNITA has said it will ''intensify its armed struggle'' against the Angolan government until it is included in talks to bring peace to Angola. South Africa is widely acknowledged to support UNITA. But some analysts wonder whether the movement may continue to pose a threat even if Pretoria withdraws support from the group. This could make withdrawal of Cuban troops from Angola difficult. UNITA wants a share in government, but the Angolan regime has so far refused to discuss a coalition.

UNITA claimed last week to have seized a diamond mine in northeastern Angola and to have kidnapped a number of Filipino, Portuguese, and British workers.

South Africa said the new SWAPO incursion was ''contrary to the letter and spirit'' of its agreement with Angola. Angola responded that if any incursion was taking place, it must have begun before the agreement - a time Angola had no control over the border region. SWAPO denies it is mounting a new incursion.

South Africa and Angola apparently reached accord on how to deal with the alleged SWAPO threat over the weekend. They also said official monitoring of the disengagement and cease-fire will begin March 1. The United States will take part if asked by both parties; it has already established a ''support staff'' office in Namibia.