South Africa's deep military penetration into Angola last week apparently is part of an effort to create a rebel-controlled buffer zone in southern Angola that would help Pretoria maintain a government to its liking in neighboring Namibia, in the view of a number of Western diplomats here, as well as the Angolan government.

South African forces have regularly crossed into southern Angola for years, ostensibly to chase guerrillas of the Southwest Africa People's Organization (SWAPO), which is fighting for independence for South African-controlled Namibia (Southwest Africa).

However, in the last year or so the pattern of some of the attacks has changed. The South African troops disrupt communications, prevent local food distribution, kill or drive away cattle and poison wells to impoverish the area, government officials have told Western diplomats. The purpose, they say, is to create discontent with the Luanda government and help gain control of the area for a Pretoria-backed Angolan guerrilla group, the National Union for the Total Independence of Angola (UNITA).

There are reports that South African troops in the south recently began giving food to the inhabitants to try to win them over to the UNITA side. Since the offensive, the South African government has talked of guerrilla activity in the area of Ndjiva, about 300 miles west of UNITA's main operating area in sparsely populated Cuando Cubango Province.

UNITA control of southern Angola, with South African support, would severely hamper SWAPO's already limited ability to cross the border for raids into Namibia. It would thus allow South Africa to gradually abandon a United Nations solution for Namibia and instead install a pro-Pretoria regime in Windhoek.

Questioned about the alleged South African approach, a Western diplomat said: "I hope it's not so, but it's difficult to find any other explanation" for the seriousness of the South African offensive in Angola, which reportedly involves about 5,000 troops.

At the same time, despite its effort to normalize relations with the United States, Angola increasingly is blaming Washington for Pretoria's military actions. Meeting after Monday's U.S. veto of a U.N. Security Council resolution condemning South Africa for its invasion, the Angolan Cabinet said it "condemns energetically the shameful connivance of the Reagan administration" with South Africa.

Even before the invasion, Luis de Almeida, a senior Angolan diplomat, said in an interview: "The United States is quietly encouraging the South Africans to attack to force Angolan concessions. The South Africans have changed completely since the Reagan administration -- they are much more arrogant."

South Africa says its troops have almost completed their withdrawal from Angola, but Luanda says the invaders still control six towns up to 100 miles inside the country.

South Africa repeatedly has maintained that its troops cross the border only in "hot pursuit" of SWAPO with no intention of engaging the Angolans. In the offensive it has emphasized Soviet-Cuban military involvement in Angola--a favorite theme of the Reagan administration--and has charged that Soviet advisers are working with SWAPO.

SWAPO rejects this charge and along with Luanda denies that it has bases in the part of the country under attack. "You can say anything and deny anything in Angola," a diplomat said.

No independent observers have access to both sides in the sparsely populated bush country. On various occasions, Angola, South Africa and UNITA have offered journalists limited tours obviously designed to support their separate cases.

Estimates of the size of the SWAPO forces are at best educated guesses. Most Western diplomats say, however, that Pretoria exaggerates the SWAPO military threat to justify its attacks in Angola. One source estimates that SWAPO has no more than 5,000 guerrillas, with only 200 to 500 operating at any one time in Namibia in a semicircular area up to 60 miles south of the border and along a 200-mile-wide strip.

Estimates of South African troop strength along the border range from a low of 20,00O to an Angolan claim, evidently exaggerated, of 45,000. "This is absurd overkill," an analyst said. "It's not a war; it's a giant crushing an ant."

There is general agreement, despite SWAPO leader Sam Nujoma's threats to fight for endless years, that there can be no military solution to the Namibian problem. Nujoma has been fighting for 15 years in what is now Africa's longest war of liberation and has made little military headway, although he has gained increasing diplomatic support.

Angola, one of the world's least-developed countries, is spending more than 25 percent of its budget on defense because of the Namibian struggle, and is facing a growing problem of displaced persons. The war is hurting Angola's development far more than Namibia's.

The alleged South African objectives would fit in with what many African officials believe is Pretoria's overall geopolitical aim: destabilization of Southern African nations to force continued economic dependence on South Africa.

UNITA attacks in Angola's central plateau have kept the strategic Benguela railroad crippled for years. The railway used to be the key outlet for vital copper exports from neighboring Zambia and Zaire. These must now go through South Africa's Indian Ocean ports. Similarly, South African-backed rebels in Mozambique have disrupted rail traffic from Zambia and Zimbabwe and forced them to maintain their dependence on the southern route.

There also are East-West ramifications of the reported South African policy. Angolan President Eduardo dos Santos has hinted that if the war intensifies he may be obliged to ask Cuban troops to join in.

The Cubans have shown no desire to enter the fray, but deeper South African penetrations could involve them. They have warned that they will retaliate if their forces are attacked. Such involvement may be one aim of some South African policy makers, since the Reagan administration might be hard put to oppose a nation fighting the Cubans.

Backing South Africa, however, would alienate black Africa and could cause a split between the United States and Europe.

South Africa and Cuba clashed once before in Angola, supporting opposing sides in the 1975-76 civil war that accompanied Angola's achievement of independence after centuries of Portuguese rule. The Marxist government of the late president Agostinho Neto called in Cuban forces to repel about 2,000 South African troops aiding guerrillas opposed to his government.

"The West is partly responsible for the Cuban presence in Angola because it failed to prevent the earlier South African intervention," a diplomat said. "Now the way to get the Cubans out is to get a Namibian settlement, but for the moment there is no chance for negotiations . . . Efforts must concentrate on cooling off the fighting."