SINCE ARMS are a means and symbol of repression, it is right that President Carter should react to South Africa's latest fit of repression by supporting mandatory U.N. sanctions against the sale of arms to Pretoria. Voluntarily, the United States has sold no weapons since 1963. South Africa's major foreign suppliers, notably France, have been reducing their sales in recent years as the international campaign against apartheid has picked up steam. On their part, the South Africans have been building up inventories, arranging hidden arms-purchase channels and strengthening their own manufacturing capabilities. All the same, as Mr. Carter declared, it was "important that we express in no uncertain terms our deep and legitimate concern." This he has done.

Other steps being taken have special angles. The senior U.S. naval attache is to be withdrawn, presumably to underline American rejection of Pretoria's contention that South Africa guards sea lanes crucial to the West. One cannot quarrel with the gesture, but it is worth observing that, as the United States heads into what will obviously be a long and difficult passage with South Africa, we should not be depriving ourselves of too many of our eyes and ears on the scene. The intelligence activity that is the essential work of military attaches can be doubly valuable as official relations sour - and as the need increases for the United States to learn how South Africa tries to circumvent the embargo. Indeed, those calling for a virtual evacuation of the American embassy to demonstrate political displeasure should consider how useful it is to have people in place to see what's going on.

The (temporary) recall of the commercial attache is also said to be in the works. This would give point to Mr. Carter's warnings about limiting private investments and official loan guarantees and other government programs. But there is a difference between hardening an arms embargo that has been in effect almost 15 years, and starting to intervene in the economic sphere - that is, in the investments, jobs and profits of American citizens. The diplomatic effect might be greater - we recently voiced our own ambivalence on this score. But here at home the political resistance would surely be greater, too. For instance: American corporations are cooperating in extending employment opportunites to South African blacks, but would they cooperate in punitive actions that had the effect of closing them out of business?

Mr. Carter says that additional steps beyond an arms embargo haven't been decided. His administration should discuss them publicly before they are decided in order to ensure the public support necessary to make the decisions work.