The United States has obtained intelligence suggesting that South Africa may have detonated a nuclear device Sept. 22, official sources said last night.

There has been no confirmation of the initial data picked up by a U.S. satellite, according to the sources. Only one of several systems for detection reported the likely nuclear explosion, and intensive follow-up efforts have not yielded additional evidence of a blast.

Nevertheless, the report has been taken with great seriousness within the government. This is both because of its source and because South Africa, which has been building a plant that can produce weapons-grade enriched uranium, is believed to have been on the threshold of nuclear weapons capability for at least two years.

The intelligence indications are a low-yield explosion estimated at 2.5 to 3 kilotons. This is the equivalent of 2,500 to 3,000 tons of TNT, about one-fourth the yield of the Hiroshima bomb.

Following the receipt of the information, European allies were informed in an effort to pool intelligence data, and a report was submitted to the Senate and House Intelligence committees.

The information was held on a highly secret basis within the government during meetings aimed at assessing the data and studying the potential purpose and possible repercussions of a South American atomic blast.

The State Department issued a carefully worded statement last night, following a report by ABC News correspondent John Scali. The official statement said:

"The U.S. government has an indication suggesting the possibility that a low-yield nuclear explosion occurred on Sept. 22 in an area of the Indian Ocean and South Atlantic, including portions of the Antartic continent and the southern part of Afria.

"No corroborating evidence has been received to date.

"We are continuing to assess whether such an event took place."

Rep. Stephen J. Solarz (D.-N.Y.), chairman of the House Foreign Affairs subcommittee on Africa, identified the area of the suspected blast last night as "off the coast of South Africa." Though the official description was imprecise, South Africa is the only prime suspect for a nuclear explosion in that area of the world.

The United States has not yet taken its suspicions to the South African government, according to the sources, in hopes of obtaining further information first. The news leak is certain to precipitate official inquiries by several governments, and is likely to bring strong reactions from black African states.

South African Foreign Minister R. F. Pik) Botha, reached by Reuter news agency in Pretoria late last night, denied any knowledge of an atomic test. "I know absolutely nothing about the matter -- why don't you ask the Russians or the Chinese or even the Americans for that matter," he was quoted as saying.

In August 1977, after reports of preparations for an A-bomb test in South Africa's Kalahari Desert and a coordinated diplomatic drive by the United States, the Soviet Union, Britain, France and West Germany, South Africa stopped test-site construction and provided assurances about its nuclear intentions.

As announced by President Carter at the time, South African authorities stated that "they do not have and do not intend to develop nuclear explosive devices for any purposes, either peaceful or as a weapon." The terms of the assurances were later disputed by then-prime minister John Vorster, however.

A South African uranium enrichment plant in the rolling countryside west of Pretoria went into pilot operation in 1977. Even before the plant was completed. French Prime Minister Raymond Barre said publicly that South Africa "already has a nuclear military capability."

If the explosion is confirmed, South Africa would become the seventh nation to set off an atomic blast. The others are the United States, the Soviet Union, China, France, Britain and India.