South Africa was barred yesterday from taking part in the 23rd annual conference of the International Atomic Energy Agency new meeting in New Delhi.

The move climaxed three years of efforts by Nigeria and other African nations opposed to Pretoria's policy of apartheld, although it fell short of their wish to eject South Africa from the agency altogether.

The expulsion from the conference, the first in the 110-member agency's 22- 22-year history, seems likely to further politicize the organization at a time when its stated purpose of promoting the peaceful uses of atomic energy as well as safeguarding against nuclear proliferation has come under increasing criticism and challenge.

The move followed by one month a bitter protest against South Africa by the Nigerian ambassador to the United Nations, of which IAEA is an autonomous agency, on the controversial "fireball" phenomenon monitored by the United States in the South Atlantic and Indian Ocean. There were initial suggestions that it was a South African nuclear explosion.

Though subsequent monitoring of fallout data by New Zealand has proven inconclusive on that incident, South Africa has Africa's most sophisticated nuclear program and is widely believed to possess nuclear weapons capability.

The Nigerian envoy at the United Nations, who assumed the Sept. 22 phenomenon was a South African nuclear explosion, demanded that nuclear cooperation with Pretoria within the IAEA be terminated acording to U.S. csources who attended the debate on the agecy's report.

The Nigerian representative also accused the West of colluding with South Africa to acquire a nuclear weapons capability.

South Africa in a skirmish two years ago lost its seat on the IAEA board of governors to Egypt. The only expulsion from membership in the agency was that of Taiwan in 1972 following a U.N. resolution recognizing the People's Republic of China.

Countries voting against yesterday's resolution, passed by a vote of 49 to 24 with 9 abstentions, included the United States, Israel, Italy and West Germany. Their spokesmen said the IAEA was not an appropriate forum to act on apartheid.

American chief delegate Gerard Smith said that the United States opposed South Africa's racial policies, but he added that this was irrelevant to whether or not the credentials of the South African delegation were in order. w

A State Department spokesman in Washington added that the United States also opposed barring South Africa because it feels Pretoria should remain under the IAEA's wing and, putatively, its influence.

South Africa's delegate at the conference, J.W.J.De Villers, called yesterday's decision illegal and politically motivated and said it would, in effect, prevent Pretoria from extending its cooperation to the agency.

The week-long conference opened Tuesday with an address by Director General Sigvard Eklund in which he pressed the agency's case for nuclear power to meet worldwide energy needs and reduce dependence on oil imports. He announced that an international conference on nuclear safety -- including "the lessons to be learned from the Three Mile Island accident" -- will be held next year in Stockholm.

Eklund, who had made a last-minute plea for South African participation, said yesterday before the vote that South Africa had agreed to report to the agency its exports of nuclear material to nonnuclear countries and might sign the 1970 Nonproliferation Treaty which IAEA administers.

South Africa is the world's third largest uranium producer and a State Department source said yesterday's resolution could mean Pretoria will pursue current nuclear negotiations supported by the Carter administration even "less