As American bankers continued to express doubts about how South Africa intends to resolve its debt problems, a leading financial official in the Pretoria government hinted yesterday at an extension to the country's four-month moratorium on foreign loan repayments.

At a news conference in Pretoria, Chris L. Stals, director general of the Department of Finance, said the freeze might not end until foreign lenders agreed to reschedule $6.5 billion in short-term debt due over the next few months, Washington Post correspondent Glenn Frankel reported.

"We would hope to lift these restrictions by the end of December, but one must be realistic and understand that the first problem is to create a structure for negotiations with overseas banks," said Stals.

American banking sources interpreted Stals' comments as an effort to gain further breathing space for South Africa, but Pretoria's failure to spell out precisely how it intends to pay back its foreign loans is already causing strains in the banking community.

Several major U.S. banks, which declined to be identified because they have big loans outstanding, said privately this week that they had severed all short-term credit covering trade links with South Africa, an important move since the country imports about $14 billion and exports $18 billion annually.

In a separate development, a crisis in the interbank market, the crucial mechanism for ensuring the smooth flow of funds among international banks, was averted this week only when the governor of the central bank, Gerhard de Kock, issued a statement pledging full government support for Nedbank, South Africa's third-largest bank.

Nedbank, whose foreign branches did not obtain an exemption from the moratorium, was reported to have had difficulty in meeting its obligations in the inter-bank market after a New York bank refused to release funds earlier this week.

Yesterday, Nedbank resumed business, and American bankers said de Kock's statement had bolstered confidence in resolving the problem.

"Things are simmering down," said one banker in reference to Nedbank.

But American bankers said longer-term problems centered on how and when international creditors would receive payment on their outstanding loans.

"Nobody knows at this stage which banks will be paid first and what the outcome of the rescheduling will be," said one American banker familiar with South Africa. "Everything is up in the air."

In London, de Kock, apparently conscious of the continuing wariness of American banks, said a continued refusal by foreign banks to lend money to the country would batter South Africa's economy and those of neighboring countries.

"There is no way you can destroy the economy of South Africa without destroying the whole of sub-Saharan Africa," de Kock said at a London news conference before flying to Frankurt for talks with officials of West German banks.

During five days of talks in New York and Washington, de Kock met with senior officials of Citicorp, Chase Manhattan, Bankers Trust and Morgan Guaranty.

The trade credit cutoff applies to the so-called letters of credit with which, for example, an American bank would guarantee payment on behalf of a South African importer of goods in return for a fee. Letters of credit grease the wheels of international trade because they give a breathing space of as much as six months to importers.

According to one banker who has regular dealings with South African companies, the market has seen a marked shrinkage of credit following Sunday's unilateral declaration of a moratorium on payment of the principal on $12 billion in loans maturing within the next 12 months.

"The South African economy is operating on a cash-only basis," the banker said.

In Pretoria, Stals said that the freeze had "scared" both exporters and potential lenders to South Africa.

"Whatever we do and whatever we arrange in the next few months, the foreign lenders have been scared by these steps, and it will take us some time to win back their confidence again," he said.