The Carter administration has withheld approval of licenses to export uranium to South Africa and plutonium to Pakistan.

Both licenses were held up because of questions of South Africa's and Pakistan's intentions to develop, produce and test nuclear weapons. South Africa is building a plant to enrich uranium it says will be used for peaceful purposes only, while Pakistan has announced that it intends to build a reprocessing plant to extract plumnium from spent uranium fuel.

The export licenses to South Africa and Pakistan were two of 12 nuclear licenses held up by the State Department to allow "further review" of the licenses. Export licenses were also held up for Bolivia, Yugoslavia, Iran, Malaysia, Bangladesh, India, Mexico, the Philippines, Taiwan and the People's Republic of China.

The licenses involving shipments to South Africa and Pakistan were the only two held up for purely political reasons. The State Department explained to Congress that the South African license was held up "awaiting the outcome of a review of U.S.-South African nuclear cooperative relationship."

State's explanation for holding up the Pakistan license was "its [Pakistan's] announced intention to obtain reprocessing capability."

The license for export to South Africa involved the shipment of 57 pounds of hight-enriched uranium bound for the Safari research reactor in South Africa. The uranium is "enriched" with 93 percent of U-235, which is the istope that fissons to protmote a chain reaction. It is the same kind of uranium used to make nuclear weapons.

The South African request for this uranium dates back more than three years. Filed during the Ford administration, it is now the oldest pending nuclear export license before the federal government.

While the hold-up of the license to South Africa is not a denial, it represents the next thing to it. The hold-up also marks what might be a disagreement inside the Carter administration about how to deal with South Africa on nuclear policy matters.

Last November, U. N. Ambassador Andrew Young said publicly that the United States should not cut off nuclear experts to South Africa. Said Young: "To cut things off would only encourage separate development of South Africa's own nuclear potential."

Owner of one of the world's richest reserves of uranium, South Africa is building its own uranium enrichment plant. Ostensibly, the plant will produce only low-enriched uranium for power but it could be beefed up to produce the kind of high-enriched uranium that goes into nuclear weapons.

There have long been rumors that South Africa plants to develop nuclear weapons. A secret site in the Kalahari Desert has been reported to be the best site for such weapons. South African statements on the matter have been ambivalent and ambiguous.

The export license held up for Pakistan involved less than a pound of plutonium, which was destined for a research reactor where it was to be used to irradiate materials with alpha particles. Plutonium can also be used to make nuclear weapons.