Ships are sunk in Mozambique harbors. Water tanks are exploded in Lesotho. Railroads are sabotaged in Angola. The southern tier of black African states sees the hand of South Africa behind these acts that have taken place with increasing regularity in recent months. They charge South Africa is out to destabilize the black-ruled nations of southern Africa politically and economically so that it can control events in the region.
Pretoria denies any complicity. A spokesman at the South African Embassy in Washington says, ''If South Africa really wanted to destabilize southern Africa, it wouldn't have taken this long to do it.'' He insists that South Africa is willing to sign nonaggression pacts with its black neighbors.
South of the political divide that separates black Africa from white-ruled South Africa, acts of sabotage are occurring, too. And South Africa is charging that it is the victim of destabilization in southern Africa.
Some United States officials take the view that when South Africa has struck directly at black states militarily it is because these states have broken a tacit understanding not to allow their territories to be used as training bases or launching pads for attacking South Africa. (The Reagan administration is more sympathetic to the South African government than previous administrations.)
What is not in doubt is that the entire region embracing the white bastions of South Africa and Namibia (South-West Africa) as well as the neighboring black states of Angola, Zimbabwe, Lesotho, and Mozambique, reverberates with calculated acts of sabotage and political violence.
In South Africa, a succession of explosions last December rocked the continent's only nuclear power plant just 25 miles north of Cape Town. And in Bloemfontein, the heartland of Afrikanerdom, the outlawed African National Congress (ANC) claimed responsibility for the February bombing of a South African government office that handles African work permits. The blast was the 30th reported incident of terrorism in South Africa since the beginning of 1982.
South African newspapers of Feb. 25 and 26 respectively reported a fire in the Atomic Energy Corporation's facility at Palindaba near Pretoria in northern South Africa and major power failures in the western and eastern Cape in the south. The attack Dec. 18-19 on the high-security Koeberg nuclear plant just outside Cape Town, where Soviet-made limpet mines were used in explosions that took place over a 12-hour period, shows that sabotage tactics are becoming increasingly more potent and sophisticated.
David Ndaba, New York spokesman of the outlawed ANC, which was banned in South Africa in 1960 and which takes responsibility for most acts of sabotage, says that so far the targets have been selective. The sabotage has been directed against symbols of apartheid (South Africa's enforced system of racial separation) such as military installations and police stations.
But Mr. Ndaba says his organization will soon be able to raise its level of activity to ''direct armed clashes'' with the South African authorities.
South Africa senses this may be true and is beefing up its military forces to meet its growing security needs. The country is already the strongest military power in Africa south of the Sahara - but its new budget calls for a 16 percent increase in military spending. And military spending tripled over the past eight years.
Much of the new defense spending will go toward a border war in Namibia, where South Africa is fighting a protracted guerrilla war with the South-West African People's Organization (SWAPO).
According to African experts like Prof. Gwendolen M. Carter, an analyst of South African politics who has recently visited southern Africa, the white republic's confidence as a regional economic and military giant is being sapped by an uneasiness over the country's internal security problems and the rise of black militancy.
South Africa's Israeli-like response to acts of terrorism is to hit hard at targets across its borders where it feels the ANC is either taking sanctuary or operating military bases.
But South Africa's assertions that it acts only in self-defense and is willing to adopt a live-and-let-live policy toward its black neighbors are questioned by the ANC and a number of Africa analysts.
''They don't attack military bases. They go for electrical power lines, fuel depots, and railroads. The aim is economic and political destabilization,'' Mr. Ndaba says.
Black Africans have long recognized that it is South Africa's hold on railroads and communications in southern Africa that is perhaps the single most important factor for keeping them under Pretoria's economic thumb. South Africa controls about 75 percent of the rail network in southern Africa.
A number of Africa experts who have visited the region echo the view that South Africa seems intent on stopping black Africa's attempts to bypass the white-ruled republic through the establishing of alternative road, rail, and sea links.
Prof. Patrick O'Meara, head of the African studies department at Indiana University, says South African direct and indirect moves against its neighbors underscore the nation's economic hegemony of the region by forcing even greater black African dependence on South Africa.
Countries as far north as Zambia and Zaire depend on South Africa's highly developed railways and ports to get out their essential copper and cobalt exports. South Africa represents 77 percent of the economic production of 10 states in southern Africa.
Says Professor O'Meara: ''By creating instability and internal upheaval in these black countries, it (South Africa) forces these countries to be preoccupied with their own internal problems and divert attention away from South Africa.''
Instability in black Africa, moreover, enhances South Africa's prospects of projecting itself as an island of white stability in a sea of black chaos.
South Africa's position is that it wants to cooperate economically with neighboring states but will not tolerate any Lebanon-like situations that would permit terrorist actions to be launched from border states. To the South Africans then, it is perfectly consistent for countries such as Malawi, Swaziland, Botswana, and Zambia to be largely off-limits to South African military action because those countries have rigorously avoided becoming active centers for anti-South African activity.
The countries that the South Africans feel pose the greatest menace to their security are Lesotho, Angola, and Mozambique.
Lesotho is surrounded by South Africa and is overwhelmingly dependent on it economically, but the tiny kingdom has demonstrated it is not a South African client state by permitting the ANC into the country. In Angola, some 20,000 to 30,000 Cuban troops ostensibly serve as a defense against South African security forces. Angola is also a staging ground for SWAPO-directed guerrilla attacks on South African-ruled Namibia.
Mozambique has no Cuban troops, but as a Marxist state (along with Angola) it poses an ideological threat to strongly anticommunist South Africa. To some extent, Mozambique is also an economic threat to South Africa, since Zaire, Zambia, and Zimbabwe would prefer to export their goods through the Mozambican ports of Beira and Maputo than through South African ports.
South Africa contends that the Soviet Union, eyeing southern Africa's vast mineral resources, is working through Angola and Mozambique, its Marxist proxies.
The South African charge against Lesotho is that the ANC headquarters there was plotting a terrorist attack against South Africa, and that the small mountain kingdom had thus broken a tacit understanding not to permit its territory to be used for terrorist purposes.
Thus while the Pretoria government denies that it is bent on a deliberate attempt to destabilize southern Africa, it has made clear its intention of attacking neighbors that support anti-South African groups such as the ANC and SWAPO.
As a result, South Africa had no qualms about having made isolated military forays into Lesotho with helicopter-borne commando troops, and into Angola with full-scale military invasions.
But Pretoria denies links with guerrilla groups that have repeatedly sabotaged sites in Lesotho and Mozambique. It also denies allegations that it is plotting insurrections against neighboring rulers hostile to its race policies. Yet there is evidence to suggest that South Africa is either openly or tacitly assisting such operations.
''All the evidence I have managed to collect indicated South Africans were involved,'' says Professor Carter, who recently traveled through Mozambique. She found it impossible to drive from Maputo to Beira, the two leading Mozambique ports, because of rebel activity.
Professor Carter says the damage in Maputo harbor had to be attributed to the South Africans because (1) the Mozambique Resistance Movement (MRM) wouldn't have the capacity to come by sea; and (2) it couldn't have done such a skilled job.
Prof. Kenneth W. Grundy of Case Western Reserve University in Cleveland, another American expert on Africa, also believes South Africa is active in the MRM campaign to harass the Mozambican authorities. He claims the MRM is operating a radio transmitter in the northern Transvaal Province of South Africa.
Mr. Grundy says a member of the South African Defense Force invited one of the professor's associates, an American, to a military training camp in the Transvaal that housed dissidents from neighboring African countries. Although his source did not divulge the origins of the Africans, Professor Grundy suspects they came from Zimbabwe, Mozambique, Angola, Zambia, and probably Lesotho.
Aside from possible support of the MRM in Mozambique, South Africa is also believed to be actively behind other antigovernment groups in neighboring black states:
Lesotho: Ntsu Mokhlekle, longstanding rival of Chief Leabua Jonathan, Lesotho's chief of state, is widely reported to be living in South Africa. His antigovernment Lesotho Liberation Army has sabotaged a number of Lesotho installations. Many African experts say the only way the National Liberation Army could operate is out of the Orange Free State province in South Africa, which lies across the Caledon River from Lesotho.
Zimbabwe: No clear-cut connection has been made between South Africa and Zimbabwe groups. Some remnants of the Selous Scouts, which fought under the white Rhodesia government of Ian Smith, are thought to be connected with the MRM in Mozambique. Prime Minister Robert Mugabe has charged South Africa with destabilizing his country. Recently three white soldiers, formerly sergeants in Ian Smith's Rhodesia Army and now enlisted in the South African Defense Force, were killed in Zimbabwe. South Africa's response was that they were not on an authorized mission.
Angola: The most tangible sign of South African collusion with African groups outside its borders is in southern Angola. During the 1975 civil war in Angola, South Africa openly came out in favor of Jonas Savimbi's pro-Western National Union for the Total Liberation of Angola (UNITA). Savimbi has not hesitated to acknowledge that he receives support from numerous countries, including South Africa.
UNITA has proved a considerable thorn in the side of the Angolan government. Even without South African assistance, it is thought that UNITA can easily hold the southern section of Angola. By occupying some of the most fertile portions of Angola, it has forced the Luanda government to become a net importer of food; the country had been a net food exporter. More strategically, UNITA has also been instrumental in disrupting the Benguela Railroad, which runs through the center of the country to the port of Lobito. The railroad was intended to divert both Zambian and Zairean exports from heading southward through South Africa.
Although Angola has been involved in military clashes with South Africa, both Lesotho and Mozambique are far more vulnerable to any South African security challenge.
Both Mozambique and Lesotho are so overwhelmingly dependent on South Africa economically that they find it prudent not to retaliate. Their response, like that of seven other African states, is to draw together in an economic alliance - the so-called Southern African Development Coordination Council (SADCC) to reduce Africa's dependence on South Africa.
But many African observers say it is precisely this attempt at economic rejection of South Africa that is instrumental in South Africa's destabilizing of these economies. If they are unstable, they cannot free themselves of their South African yoke.
As evidence of this, African states point to the explosion in Maseru, Lesotho , on Jan. 27 of this year, which coincided with the opening of the annual SADCC meeting. Two 40-foot-wide concrete water tanks and a nearby pumping station just one mile from the South African border were struck by explosives planted by the Lesotho Liberation Army.
In his opening speech, the conference chairman, Botswana Finance Minister Peter Mmusi, said the attack fitted a recent pattern of what he called '' the continuing, indeed escalating, acts of economic destabilization which are being directed at our member states.''
''It is not much use,'' he said, ''to develop ports and pipelines, roads and railways, and then watch in silence as they are blown up.''