High U.S. officials expressed uncertainty and some puzzlement yesterday about the cause of a mysterious event near South Africa five weeks ago today that registered on a nuclear detection satellite as an atomic explosion.

"It is not clear there has been a nuclear detonation," Secretary of State Cyrus R. Vance told a news conference in Gainesville, Fla. But at the same time, Vance and others gave new details of the satellite sighting, which has caused great concern in high circles that another nation has joined the nuclear weapons club.

The sighting by a U.S. Vela satellite about 60,000 miles in a space was of two bright flashes, or pulses, in a sequence and timing characteristic of an atomic exposion.

According to Sen. John Glenn (D-Ohio), the former astronaut who is chairman of a Senate subcommittee on nuclear weapons proliferation, Vela satellites have picked up signs of atomic exposions in the atmosphere 41 times since having been sent into space in 1963, and each of those 41 was later confirmed to be an atomic weapons test.

In this case, the sighting was about 3 a.m. Africa time on Saturday, Sept. 22, a cloudy night. The bright flashes were somewhere in a large area of the Southern Hemisphere, a circle with a radius of about 3,000 miles including Antartica, the southern part of Africa and lots of ocean. The slighting was transmitted electronically to the Air Force Technical Applications Division at Patrick Air Force Base near Cape Canaveral.

The air force personnel watched a stylus draw two big blips, representing successive flashes of light in the familiar atomic weapons signature. Within two hours of the event, which happened around 10 p.m. Sept. 21 Washington Carter, Secretary of State Cyrus R. Vance, Secretary of Defense Harold Brown and other top officials.

The immediate suspect, and still the most likely in some official minds, was South Africa, the only nation in the area believed capable of a nuclear explosion.However, there had been no hint of an imminent South African blast and there was no announcement by that country that it had tested a nuclear device.

In addition to the Vela satellites scanning large portions of the earth and outer space, the United Sates employs several other highly sophisticated means to gather nuclear explosion data, including a fleet of airplanes to sample the air for radioactive materials and debris, seismic stations to record the telltate jarring of the earth and acoustic devices to listen for the sound waves set off by a nuclear blast.

Intensive searching, using these other means, failed to turn up confirmation of the report by the Vela satellite. In early stages what are described as "squiggles" on both seismic and acoustic devices seemed to describe an explosion at the right time and area, but further study described the fluctuations as not significant.

Based on the satellite information, government experts came up with an estimate of 2.5 to 3 kilotons (equivalent to 2,500 to 3,000 tons of Tnt) as the probable power of the blast involved. Such an atomic expolsion would have caused an extremely bright flash in the sky, though somewhat less than the fireball oat Hirohima, where the bomb was about 12 kilotons and was later described as "brighter than a thousand suns."

According to official sources, neither last month nor since have there been any reports by people who saw the flashes in the sky. U.S. intelligence sent out discreet feelers for such reports as well as for any unusual statements or movements in nations which might have been involved, so far without avail.

The lack of technological or physical confirmation has not been taken as conclusive, for several reasons. A seniro Pentagon official, in a briefing for reporters, said that in view of the relatively low yield of the apparent explosion and the remoteness of the area, it would not necessarily be expected that radioactive sampling, seismic or acoustic devices would provide confirmation.

The hour of the night, the heavy cloud cover and the remoteness of the area may have combined to limit physical observation, according to the official.

In addition to a nuclear expolsion from a still undetermined source, government officials are considering two other possibilities:

That a combination of natural events, such as lightning flashes in sequence with a meteor, for example, could have resembled the nuclear weapons signature on the Vela satellite. No one such event, but a combination of events, in extraordinary and precise sequence, would have been necessary for this to happen.

That somehow the Vela satellite, which has been extremely accurate in the past, malfunctioned and printed the record of an event that never happened. tA scientific board of experts who were not involved in the manufacture or management of the satellite is being convened to study this possibility.

Three diamond-shaped Vela satellites orbiting the earth once every 4.5 days at an altitude of 60,000 to 70,000 miles keep a watchful eye for nuclear detonations in the atmosphere or outer spaces. In addition to optical sensors to register ligt, the satellites were built to pick up x-rays produced by an atomic expolision and "Electro-magnetic pulses," radio signals of very short duration which are peculiar to nuclear expolsions.

The Vela satellite that recorded the Sept. 22 event was launches in 1970, according to the Air Force, and is the newest of the type. Some of its sensors are woprn out and no longer function, and others are not sensitive enough to pick up such a small blast, an Air Force spokesman said.

According to the senior defense officials, the only information the government has is "one piece of data," the light curve of two flashes in the characteristic nuclear weapons pattern.

If an unannounced, unackowledged nuclear explosion did occur, as the consensus of officials suggests, the questions of who and why are matters of grave importance for governments and people everywhere.

Most nations detonating their first atomic blast have started with much larger and thus less complex explosions, about 10 to 20 kilontons. But the senior Pentagon official said it is quite possible for a first nuclear explosion to be made deliberately much smaller, presumably in an effect to avoid detection.

Some scientists said the believe the explosion was conducted over the ocean in an attempt at concealment. One scientist speculated that this may not have been the first test over water in this remote area, possibly by South Africa, but that it is only the first detected by the United States.

South Africa has been the prime suspect because of the area of the apparent blast and South Africa's reported ability to enrich natural uranium to weapons grade. The Pretoria government apparently approached a weapons test in the summer of 1977, but backed off after unprecedented teamwork warnings by the United Sates, the Soviet Union, Britain, Frances and West Germany.

A secret atomic blast might assure South Africa of a doomsday capability. An acknowledged blast would bring down the wrath of its Africian neighbors, cause a crisis with the great powers and probably lead to the worldwide economic boycott its enemies long have sought.

It is possible that another nation secretly brought a nuclear device into the area, perhaps on a boat, which was then abandoned for use as the launch platform. Ship traffic has been investigated, so far without results.

Among the nations openly discussed here yesterday in rampant speculation about the source of the nuclear blast were Israel, which is believed to have a small number of untested plutonium bombs, and the "near-nuclear" nations of Pakistan, Taiwan, South Korea, Brazil and Argentina.

Officials have all but ruled out the possiblilty that a nuclear submarine exploded in the area. A reactor accident would not have caused the flashes of light, officials said. The accidental explosion of a nuclear weapon aboard a submarine seems unlikely, because no subs are believed to have been in the area.

There is always the possibility that the explosion, if it was an explosion, was detonated by the Soviet Union or China to test something new. But officials have no explanation why they would do so in such a faraway place, since neither nation has been shy before about weapons tests on its own testing grounds.