Leading American banks have severed key short-term credit links covering trade with South Africa following Pretoria's declaration of a four-month debt moratorium, authoritative banking sources said yesterday.

The credit cutoff, by all indications a technical reaction, nevertheless threatens to disrupt trade between South Africa and the United States and is a severe blow to the white minority government's efforts to restore confidence in its beleaguered economy.

South Africa imports about $14 billion worth of goods and exports $18 billion annually. Any disruption of short-term credit would damage the country's economy severely.

American banking sources said that the credit block applied to South African importers seeking short-term financing for goods from the United States. They described it as an "understandable response" to South Africa's unilateral declaration Sunday that it would halt payment of principal on $12 billion in short-term loans falling due within 12 months.

"Bankers are very conservative people," one banking source said. "If someone says they are not going to pay back their loans, then they get very nervous and stop putting up new credit."

In London, the governor of South Africa's central bank, Gerhard de Kock, in an apparent effort to steady nerves, said that payments between banks and businesses would continue "in accordance with normal market practice."

He also pledged that the central bank would stand behind the country's third-largest national banking group, Nedbank, which reportedly has experienced difficulties in meeting its obligations, yet another factor in American bankers' extreme caution toward South Africa.

The hard-nosed commercial response by American bankers to South Africa's financial crisis has been reinforced by increasing opposition within South Africa to the country's system of racial segregation, which in turn has led to international political pressure.

Leading American banks contacted yesterday refused to comment officially on the credit cutoff, but several well-informed banking sources said that it centered on "letters of credit," the vital means banks use to grease the wheels of international trade.

Under the system, for example, a South African company wishing to import $100,000 worth of oil from a major U.S. oil company would seek a letter of credit from an American bank. This letter would guarantee payment of $100,000 by the bank within a certain time, and, upon receipt, would trigger release of the oil shipment to South Africa. The South African importer then would pay the American bank the $100,000 plus a fee for putting up the guarantee.

Several leading U.S. banks, which refused to be identified, said unofficially yesterday that they had halted all such letters of credit. "No U.S. bank is issuing letters of credit until it gets a guarantee from the governor of the South African central bank saying he will make good that letter of credit," one authoritative banking source said.

De Kock left the United States Tuesday after five days of talks with monetary authorities and commercial banks, including Citicorp, Bankers Trust, Chase Manhattan and Morgan Guaranty. Banking sources said that the governor had been far from clear on how he proposed to deal with the letters of credit, one of the unresolved issues arising from the debt moratorium.

Last Sunday, however, South African Finance Minister Barend du Plessis, declaring the reimposition of foreign exchange controls and the debt moratorium, said that no restrictions would apply to payments for imports. This would appear to apply to letters of credit, but banking sources said yesterday that this was far from clear-cut.

The uncertainty arises from how South Africa proposes to deal with past, present and future letters of credit under terms of its moratorium, to be announced shortly. According to one banker, de Kock is expected to say that South Africa will spread repayments over a two-year period after the moratorium.

"The question is whether the letters of credit will be inserted into that two-year period," he said, "which means a lot of bankers will be kept waiting for their money."

Another banker said: "What you have is a picture of total confusion. Until that is resolved, people are going to hedge their bets by not renewing letters of credit or offering new credit lines."

The situation is further confused by South African banks' reliance on credit lines from other international banks to support South African exporters. This "interbank" market relies on absolute confidence that each bank can match its liabilities with its deposits.

Earlier this week, the interbank market was thrown into confusion by the failure of Nedbank to obtain an exemption from the moratorium for its foreign offices. Other banks, including Volskas, Trustbank and a subsidiary of France's Banque Indosuez, have secured permission from the central bank to operate normally abroad.

There was feverish speculation in New York Wednesday that Nedbank had had difficulties in the interbank market. De Kock issued a statement yesterday that the South African Reserve Bank would "assist Nedbank in bringing about an orderly disposition of its foreign exchange obligations and meeting liquidity needs of its overseas obligations." Throughout the uncertainty, Nedbank has maintained that its problems were solely due to the debt freeze, which meant it could not release funds.

This statement was seen in the banking community as an effort by South Africa to reassure bankers who have been ruffled by the lack of detail about the moratorium.