President Carter officially announced yesterday that the United States will convert its voluntary ban on arms sales to South Africa into a broadened formal embargo, along with most members of the United Nations.

This action will demonstrate "our deep and legitimate concern" about South Africa's suppression of black-conscioucness leaders and organizations, he told a televised news conference. The limited U.S. move stops short of a far more damaging economic embargo on South Africa, leaving a door open for diplomacy.

The President also rolled back from his recent claims that the United States and the Soviet Union are within "a few weeks" of a new nuclear arms control pact. What might emerge by then, he said, are "general terms for a settlement." with "a long and tedious negotiation" still ahead over "the details."

The action which the Carter administration plans to take against South Africa would expand the voluntary embargo on arms sales to that nation that has existed since 1963. In addition to banning military space parts as mentioned by the President yesterday the administration also intends to embargo executive-type aircraft computers and perhaps other items in the "gray area" between civilian and military equipment.

In addition it was learned, Carter plans to withdraw the senior U.S. naval attache and the commercial attache from the American embassy in South Africa, and launch a full review of U.S.-South African relations.

These actions, in turn, are tied into diplomatic bargaining in the United Nations. In recent years, the United States and other Western nations have vetoed black African demands for total economic sanctions against South Africa. Now, by supporting more moderate action, the Western nations hope to put some new pressure on South Africa, while avoiding upsetting other diplomatic efforts in Africa in which they need that nation's cooperation.

Carter referred to these dual interests in his news conference. He said the United States still hopes that South Africans "will not sever themselves from the rest of the world," and "will cooperate with us in bringing peace "to Namibia (Southwest Africa), which South Africa now rules and to Rhodesia, both now governed by white minorities.

"The crisis," as Carter put it, was touched off last week when South Africa "eliminated many of the organizations" seeking racial equality in that nation, closed down the two leading black newspapers, and arrested many leading opponents of its racial apartheid system.

As a consequence, Carter said "it is important that we express in no uncertain terms about those actions of South Africa."

Carter said he did not agree with claims that the United States is now "meddling" in South Africa's internal affairs. To deplore "blatant depreivation of basic human rights" in South Africa or other nations, he said, "is proper." While the United States has condemned South Africa's racial apartheid, he said. "We have never laid out any specific action they should take nor any time schedule . . ."

The United States never was a large arms supplier to South Africa. France is now a main source of arms for South Africa, and Israel is also a supplier, Britain voluntarily ended sales of outright military hardware, along with the United States, in 1963.

In the President's comments yesterday on the U.S. Soviet strategic arms limitation talks (SALT), he was turning away from extremely optimistic claims which conflicted with the actual state of negotiations.

Carter, in Des Moines last Friday, said, "In a few weeks my prediction is that we will have a SALT agreement that will be a pride of our country . . ."

In Washington, officials were talking in terms of months or declining to predict at all. When reporters in Des Moines questioned the president's optimism. Carter through spokeman Jody Powell added the qualification that "I don't know how many weeks" would be required for an accord.

Next day, in Los Angeles, Carter acted another amendment, saying he believed that "in a few weeks we will be able to announce some success in our negotiations . . ." Yesterday he drastically cut back that statement to: "I would guess that we have a fairly good prospect within the next few weeks of a description of the general terms for a settlement. The details, the exact procedures . . . would take a long and tedious negotiation."

In addition, in another display fo caution over ongoing negotiations, Carter yesterday avoided reference to a "homeland" or "legitimate rights" for Arab Palestinians, in discussing the Arab-Israeli dispute.

Carter instead used the more neutral terminology of equal rights for "human beings," including "the Palestinians" and "Jewish refugees." This follows open alarm by American Jewish leaders, who charge that the Carter administration is tilting against Israel in trying to produce a new Arab-Israeli conference on a Middle Ease peace settlement.