President Carter said yesterday he has made a decision on whether to support United Nations sanctions against South Africa but he declined to disclose it.

The U.N. is still debating the issue of imposing arms and trade embargoes against the South Aficans.

While keeping his own counsel on the American position, Carter made plain that he hopes to avoid a complete rupture with the South African government.

"I haven't had much experience with sanctions yet," he said, "but I think there are all varying kinds - sanctions against the sale of weapons and all degrees of economic sanctions. But what we want is to have a resolution of the South African threats against peace in Rhodesia and Namibia and also of course have the rights of the South African people . . . protected. And we'll try to modify our position to bring about all three of these things.

"We're trying to coordinate our position with not only our European and western allies but the friends we have among the leaders of the African nations and I think we've made the right decision."

That decision, the White House said later, will be announced at "the appropriate time."

The U.S. ambassador to the United Nations, Andrew Young said on Monday that he personally favors some form of sanctions but he stressed then that he was not voicing administration policy.

Possible sanctions range from a mandatory arms embargo to a worldwide ban on trade with South Africa. Opinions vary on what effect such sanctions would have. The United States and Britain, for example, have observed a voluntary arms ban for years. A mandatory U.N. arms embargo would primarily affect Israel and France. Pretoirai's major contractual suppliers.

As for trade generally, the United States was the largest single supplier to South Africa last year, selling it $1.35 billion in imports.

William Bowdler, U.S. ambassador to South Africa, whom Carter recalled last week, spent yesterday in a series of State Department meetings "on a full range of our relations with South Africa," a department spokesman said.

Bowdler "will be going back to Pretoria shortly," spokesman John Trattner added, but no date has been set.

Meanwhile, Sen. Dick Clark (D. lowa), chairman of the Foreign Relations subcommittee on African Affairs, said in an interview that the United States "should take the lead in working for a mandatory arms embargo in the U.N."

A State Department source said such an embargo might not have much effect "because the South Africans say they are 75 per cent self-sufficient in arms, and they are completely self-sufficient in the kind of arms they would need - machine guns, fifles, armored personnel carriers - to fight any insurrection of their own people."

Clark, however, said an arms embargo "would be a significant step - a very important signal that they are being isolated, and that if they don't make changes, they will find themselves increasingly isolated."

The senator said that if the United States were to seek economic sanctions at the United Nations, "Britain would not stand for it. We don't want to get in the position of being vetoed. An economic embargo should be a last step rather than a first step. We want to leave room for them to respond."

Clark also said the United States should take several unilateral actions. "We should discourage any further U.S. investment or bank loans to South Africa. We should end our Export-Import Bank guarantees of loans to U.S. firms selling goods to South Africa. And we should withdraw our commercial and defense attaches."

The book value of U.S. direct business investment in plant and equipment in South Africa totaled $1,665 billion last year. But in new capital outflows from U.S. businesses, the figure was relatively small - $9 million of the firms own money.

In previous years Britain was the largest single exporter to South Africa. Part of the surge in U.S. exports resulted from a $185 million sale of five Boeing 747 jets last year. The bulk of U.S. sales there are industrial machinery, all kinds of mechanical appliances from convey or systems to values, electrical equipment and chemical and paper products.