South Africa has performed extensive research on nuclear weapons and had sufficient fissile material in hand to create an atomic blast more than a decade ago, according to declassified Central Intelligence Agency documents released yesterday.

The heavily censored documents, which appear to provide the first official U.S. confirmation of the South African effort, state that South Africa "apparently suspended preparations" to test a nuclear weapon in 1977 because of "strong U.S. pressure and other international reactions."

The 1979 and 1980 CIA reports also lend credence to longstanding reports of collusion between South Africa and Israel on military matters. "Israelis have not only participated in certain South African nuclear research activities over the last few years, but they have also offered and transferred various sorts of advanced non-nuclear weapons technology to South Africa," one report stated.

Cleared portions of the documents do not reach conclusions, however, about possible South African and Israeli responsibility for a mysterious Sept. 22, 1979, "flash" detected by a U.S. surveillance satellite over the South Atlantic.

"Technical information and analyses suggest" the flash was caused by a nuclear explosion with a force of less than three kilotons, equivalent to roughly 3,000 tons of TNT, one report said. "On the basis of available information, we cannot determine with certainty the nature and origin of the event."

The report, obtained by the Natural Resources Defense Council (NRDC) under the Freedom of Information Act, noted, however, that special security precautions were taken at a South African naval port during the week of the blast, which "could have screened sensitive loading or unloading operations as well as ship movements."

It also said a South African naval unit that included a search-and-rescue team was placed on alert then, and it cited statements by former South African prime minister P.W. Botha alluding to nuclear weapons progress shortly afterward.

The NRDC said the reports raise questions about South Africa's recent pledge to sign the global Nuclear Non-Proliferation Treaty barring nuclear weapons development and requiring foreign inspections of key nuclear-related facilities.

"Even if Pretoria placed all nuclear production facilities under international safeguards . . . foreign specialists would reason that a previously amassed secret stockpile of weapons-grade uranium probably was being maintained," or that such material could be produced on short notice, the 1980 CIA report stated.

The NRDC noted in releasing the reports that the U.S. Customs Service last week shut down a Miami company for alleged illegal shipments to South Africa of components used in "guided missile delivery systems for standard and nuclear weapons," according to a government affidavit.

NRDC senior staff scientist Thomas B. Cochran called on South Africa to make "a complete break from its previous nuclear weapons program and {provide} a complete account of all past activities. Otherwise, it would make a mockery of the . . . {treaty}."

South African Embassy spokesman Coenraad Bezuidenhout said he had not seen the CIA documents and could not comment.