U.S. officials are convinced that the recent mysterious shipments of enriched uranium for South Africa's nuclear program originated in China, which is anything but a friend of the Pretoria regime.

The surprise shipments, made public early last week without identification of the source of supply, are expected to enable South Africa to begin operation of two 1,000-megawatt atomic power plants without relying on U.S. nuclear fuel.

The Chinese uranium was supplied through a Swiss firm acting as a middleman, or "cutout," to mask the origin of the uranium, the officials said.

The United States had been using South Africa's need for atomic fuel as a bargaining chip in the drive to induce Pretoria to agree to international inspection of its nuclear facilities. The sudden acquisition of enriched uranium from another source effectively undercut the U.S. pressures.

While a Chinese uranium supply for South Africa is the most dramatic incident to come to light, it would not be the first Chinese shipment of nuclear material abroad that seemed to complicate the U.S. drive against the spread of atomic weapons.

In an earlier transaction, Peking reportedly supplied Argentina with heavy water for atomic plants as well as low- and medium-enriched uranium. Argentina and Brazil have been the center of U.S. nonproliferation efforts in Latin America. The Chinese action brought a strong but private protest from the United States, sources said.

Sources here said the commercially astute Chinese seem to see little difference between sales of nuclear materials, including enriched uranium, and sales of most other commodities on the international market. Due to the large output of China's gaseous diffusion plants, which produce material for its weapons program, Peking is reported to have at least a temporary surplus of enriched uranium. That is worth a lot of money on the world market, especially from states seeking to avoid the strict regulation imposed by the United States and other suppliers.

A Nuclear Regulatory Commission official, who asked not to be named, said the Chinese sale to South Africa would be "an explosive piece of work," if confirmed. "The Chinese are very much involved in African affairs, and they certainly would not want it known that they are supplying South Africa," the official said.

A Chinese Embassy spokesman said reports of a sale to South Africa were "a sheer fabrication." The spokesman said: "We have no diplomatic relations with South Africa and no trade relations whatever. If we knew where the material was going, we would never sell it to South Africa."

According to U.S. experts, light water reactors of the type South Africa is building require about 30 tons of fuel a year. One source estimated the surprise supply of enriched uranium, being fabricated into fuel rods for South Africa by a French firm, at about 50 tons, almost enough to operate South Africa's two reactors for a year. South Africa is well along in developing its own facilities for enrichment of natural uranium. While the plant is believed to have sufficient capacity to make enriched uranium for at least one atomic bomb, it is not believed capable of initially supplying two new power reactors. It is possible, though uncertain, that the reported Chinese shipment could meet South Africa's pressing needs until its own enrichment plant is working well enough to supply the reactors, sources said.

A problem for South Africa is a previous contract with the United States to buy its enriched uranium here. According to the Energy Department, the United States is holding $29 million worth of enriched uranium on South Africa's account and has a contract to supply another $340 million worth.

The reported Chinese sale, on the heels of the similar incident involving Argentina, has raised serious U.S. concern that Peking could be a wild card on the international proliferation scene, making the effort to stop the spread of nuclear weapons even more tenuous and difficult.