Preisdent Carter said yesterday that South Africa has informed the United States that "they do not have and do not intend to develop nuclear explosive devices for any purpose, either peaceful or as a weapon . . ."

Reports that South African was on the verge of blasting its way into the world's exclusive nuclear club, first publicized by the Soviet news agency Tass on Aug. 8, have spread alarm among many nations.South Africa has indignantly denied the reports.

From American and French sources it was learned that suspicions about a potential South African nuclear test were aroused by American and Soviet Satellite reconnaissance sightings of activity in the Kalahari Desert region of Suth Africa, near the Southwest African (Namibian) border. One French radio report claimed that atomic test sites were spotted in the desert. American sources declined to be specific about what was sighted from space.

President Carter made the first official public referrence yesterday to the suspicions raised in the Kalahari Dregion, which for two weeks have preoccupied intelligence specialists in the United States, France, Britain, West Germany and other countries, in addition to the Soviet Union.

South Africa, he said, has informed the United States "that the Kalahari test site which has been in question is not designed for use to test nuclear explosives, and that no nuclear explosive test will be taken in South Africa now or in the future."

Carter first said, "We appreciate this commitment for South Africa," and then referred to it as "this information." Other administration sources said that the United States has "assurances without equivocation" that South Africa will not launch a nuclear test blast.

The United States, Carter said, nevertheless will "continue to monitor the situation there very closely," in addition to urging South Africa to place all its nuclear power facilities under international safeguards, and to signthe 1963 nuclear non-proliferation treaty.

Carter made this additional points on foreign policy at his news conference:

The United States has assurance from Panama in the two new treaties being negotaited that it will be "involved directly" if any new sea-level canal should be built to replace the existing waterway. This had previously been disclosed. Carter evidently wanted to underscore it as one of the benefits from the controversial treaty negotiations.

A meeting scheduled in Lusaka, Zambia, on Saturday with British Foreign Secretary David Owen, American Ambassador to the United Nations Andrew Young and the presidents of the "front-line" African states, is an "encouraging" step toward resolving the black-white conflict in Rhodesia. Owen and Young will present the new Anglo-American proposals to the leaders of black Africa.

The American protest to Isreal last week that three new settlements it plans to build on the West Bank of the Jordan River "are illegal" represents "the extend of our intention" at this time. Carter said, "Obviously, we could exert pressure" on Israel to reinforce the protest, but will not so now because Israeli prime Minister Menahem Begin has said future negotiations "would include these areas."

In an obvious counter to reports that Secretary of States Cyrus R. Vance is receiving downgraded attention by the Chinese leadership in his visit to Peking. Carter said that Vance last night was attending a banquet given by Teng Hsio-ping, a vice chairman of the Chinese Communist Party, who is also deputy prime minister. Carter said Vance is holding "indepth" talks with China's leaders.

The controversy over South African's possible plans for nuclear tests provoked no follow-up questions at the President's news conference, but the issue has aroused a furor in Europe and in Africa.

If South Africa were to set off a nuclear blast, it would raise the risk that other nations would plunge across the nuclear divide, as India did in 1974 with a "peaceful nuclear explosion." In addition, it would put on the spot, in southern Africa's volatile black white struggle, every white nation that is engagged in business for other activity in white-ruled South Africa.

That undoubtedly is why the first charge that south Africa is preparing "for the creation of the nuclear weapon" came from the Soviet Union, followed by a soviet charge that plutonium for the bombs would come from two atomic power plants that France has agreed to supply to South Africa.

French Foreign Minister Louis de Guiringaud publicly warned Monday that South Africa is preparing "an atomic explosion that the South Africans assert will be peaceful," but if it proceeds it will face "serious consequences" in its relations with Frence. Such an act, Guiringaud said, would "imperil all peace efforts in southern Africa."

French sources went further yesterday, intensifying the warnings of grave repercussions on relations with South Africa if it proceeds, despite South Africa's denials over the weekend and again Monday that it had any such intentions.

South African Foreign Minister R. W. (Pik) Botha, in a new disclaimer yesterday, said that if the French minister "wants to, he can reject my denial - that's his prerogative . . . He can suffer from illusions if he wants to; but he cannot attack something . . . which we are not doing."

The State Department over the weekend said any nuclear test by South Africa would carry "serious implications" Britain said it would be "an extremely grave affair." West Germany registered similar alarm, along with Holland and other nations.

The United States has nuclear technology arrangements with South Africa, which is one of the world's largest producers of uranium. According to the South African embassy, only one American-supplied nuclear reactor, for research and materials testing, is operating in South Africa.

Carter's news conference was dominated, however, in the foreign affairs area, by the Panama Canal treaties dispute, the current American preocupation.

The Veterans of Foreign Wars convention in Minneapolis yesterday shouted seemingly unanimous opposition to the pending treaties, which would turn over control of the water-way to Panama in the year 2000. The VFW resolution denounced the plan as "a political, psychological time bomb . . . consciously fabricated by the revolutionary government of Panama . . . to explode to the detriment of the United States and the world shipping industry."

Carter told his news conference that one reason why "there is not popular support for the Panama Canal treaty at this point" . . . probably "a great deal of misconception about what is being concluded in Panama . . ."

The President said he "was not convinced . . . a year ago" that the treaties would be advantageous to the United States. But now, he said "the terms that we hope to achieve . . . have all been achieved."

He said "I think most of the objections that were raised earlier about a giveway, of highly exorbitant payment to Panama, loss of control, takeover by some other government, a prohibition against the free use of the canal, all those questions which were legitimate in the past have now been answered successfully . . ."

The White House yesterday began a series of intensive briefings of leaders around the country to gain support for the canal treaties, which face an uphill battle for Senate ratification. Administration officials conducted a two-hour briefing for about 40 political and civic leaders from kentucky and Mississippi and Gov. Cliff Finch of Mississippi and Gov. Julian Carroll of Kentucky.