The South African Navy today began investigating the possibility that an unexplained low-yield nuclear blast spotted by a U.S. surveillance satellite was actually an "accident" on board a Soviet nuclear submarine, naval headquarters here said.

The chief of the Navy, Vice Adm. J. C. Walters, termed this a "real possibility." In a short statement to the South African Press Agency, Walters said, "It is a matter of common knowledge that there was during the period of September, for instance, a Soviet Echo II class nuclear submarine in the vicinity of the strategic passage around the Cape of Good Hope."

U.S. officials in Washington said yesterday they doubted that a nuclear submarine exploded in the area. They said a reactor accident would not have caused the flashes of light picked up by the U.S. Vela satellite. The officials added that an accidental explosion of a nuclear weapon aboard a submarine seemed unlikely because no subs were believed to have been in the vicinity.

The Vela satellite could have recorded the accidental detonation of a nuclear weapon, but not a mishap involving a reactor such as those that power nuclear submarines, since nuclear reactors do not explode.

The South African investigation of a possible nuclear submarine accident aroused some skepticism in Washington among observers who suspected South Africa was trying to divert scrutiny from itself.

Walters' statement follows a denial yesterday by the head of the South African Atomic Energy Board that this country had detonated a nuclear device. U.S. administration suggestions to this effect were called "complete nonsense" by J. Wynand De Villiers.

The U.S. government announced late Thursday that a surveillance satellite had indicated a low-yield blast of 2.5 to 3 kilotons, equal to one-fourth the power of the Hiroshima bomb, had occurred Sept. 22 in an area that includes part of the Indian and southern Atlantic oceans as well as southern Africa and the Antarctica. The United States is attempting to establish corroborating evidence of the explosion, which so far has only been detected by one of several American nuclear explosion monitoring systems.

U.S. officials immediately suspected South Africa had tested a nuclear device since its nuclear energy program is in an advanced state and Pretoria has stalled on signing the Nuclear Nonproliferation Treaty, pointing out that other potential atomic powers also have not signed it.Concerns about nonproliferation were enhanced by South Africa's decision to build a uranium enrichment plant that may be able to produce weapons-grade material. The facility is now under construction.

A South African Defense Department spokesman refused to elaborate on Walters' statement, but in all likelihood the investigation launched today will be conducted at South Africa's sophisticated underground maritime communications and surveillance center, Silvermine, a few miles from South Africa's principal naval base at Simonstown.

At the Silvermine complex, 20 million square miles of ocean are monitored by sophisticated electronic devices that chart the movements of every ship sailing around the Cape, according to government literature.

Silvermine not only pinpoints the locations of all the ships, the releases say, but identifies them according to country of origin, type and armaments on board. Its tracking system is connected to NATO's surveillance system, according to reports issued by progovernment foundations here.